Electrolyzer  and method of use

ABSTRACT

Disclosed are membrane electrode assemblies having a cathode layer comprising a carbon oxide reduction catalyst that promotes reduction of a carbon oxide; an anode layer comprising a catalyst that promotes oxidation of a water; a polymer electrolyte membrane (PEM) layer disposed between, and in contact with, the cathode layer and the anode layer; and a salt having a concentration of at least about 10 uM in at least a portion of the MEA.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 62/772,460, filed on Nov. 28, 2018 and U.S. Provisional ApplicationSer. No. 62/939,960, filed on Nov. 25, 2019, which are incorporatedherein by reference in their entireties and for all purposes.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Award NumberNNX17CJ02C awarded by the National Aeronautics and Space Administrationand Award Number DE-AR0000819 awarded by the U.S. Department of Energy(ARPA-E). The government has certain rights in the invention.

TECHNICAL FIELD

This disclosure relates generally to the electrolytic carbon oxidereduction field, and more specifically to systems and methods forelectrolytic carbon oxide reactor operation.

BACKGROUND

Electrolytic carbon dioxide reactors must balance various operatingconditions such as reactant composition at the anode and cathode,electrical energy delivered to the anode and cathode, and the physicalchemical environment of the electrolyte, anode, and cathode. Balancingthese conditions can have a strong impact on the electrolytic reactor'soperating voltage, Faradaic yield, and mix of products generated at thecathode, including carbon monoxide (CO) and/or other carbon-containingproducts (CCPs) and hydrogen.

Background and contextual descriptions contained herein are providedsolely for the purpose of generally presenting the context of thedisclosure. Much of this disclosure presents work of the inventors, andsimply because such work is described in the background section orpresented as context elsewhere herein does not mean that such work isadmitted prior art.

SUMMARY

On aspect of this disclosure pertains to membrane electrode assemblies(MEAs) that may be characterized by the following features: (a) acathode layer comprising a carbon oxide reduction catalyst that promotesreduction of a carbon oxide; (b) an anode layer comprising a catalystthat promotes oxidation of a water; (c) a polymer electrolyte membrane(PEM) layer disposed between, and in contact with, the cathode layer andthe anode layer; and (d) salt ions from a salt solution that contactsthe MEA, wherein the salt in the salt solution has a concentration of atleast about 10 uM. The MEA that contacts the salt solution may have aconcentration of the salt (or ions of the salt) that deviate from theconcentration of salt in the salt solution. In some embodiments, theconcentration of salt or salt ions (accounting for multiple counterionsdonated by a multivalent ion) in the MEA is less than the concentrationof salt in the salt solution.

In certain embodiments, the carbon oxide is carbon dioxide and thecarbon oxide reduction catalyst comprises gold, silver, copper, or acombination thereof. In certain embodiments, the carbon oxide is carbonmonoxide and the carbon oxide reduction catalyst comprises gold, silver,copper, or a combination thereof.

In certain embodiments, the cathode layer comprises an anion conductingpolymer. In certain embodiments, the anode layer comprises a cationconducting polymer.

In certain embodiments, the MEA is bipolar, having at least one layer ofa cation conducting polymer, and at least one layer of an anionconducting polymer. In some implementations, the PEM layer comprises apolymer electrolyte layer and a cathode buffer layer. As an example, thePEM layer may include a cation conducting polymer and the cathode bufferlayer comprises and anion conducting polymer. In some cases, the PEMlayer comprises an anion conducting polymer.

In certain embodiments, the salt ions comprise alkali metal ions. Insome cases, the salt ions comprise an anion selected from the groupconsisting of phosphate, sulfate, carbonate, bicarbonate, and hydroxide.

In certain embodiments, the MEA is a bipolar MEA, and the carbon oxidereduction catalyst comprises copper. In some such cases, the saltcomprises (i) an alkali metal cation, and (ii) a bicarbonate, a sulfate,or a hydroxide anion. Such salt may be present in the salt solution at aconcentration of about 1 mM to about 1M, or about 1 mM to about 50 mM.

In some cases, a bipolar MEA is configured to produce methane byreducing carbon dioxide and/or carbon monoxide at the cathode layer, andwherein the salt ions are sodium ions. In some cases, the bipolar MEA isconfigured to produce one or more organic compounds having two or morecarbon atoms by reducing carbon dioxide and/or carbon monoxide at thecathode layer, and wherein the salt ions comprise ions of potassium,cesium, rubidium, or any combination thereof.

In certain embodiments, the MEA is a bipolar MEA, wherein the carbonoxide reduction catalyst comprises gold, and the salt comprises (i) analkali metal cation and (ii) a bicarbonate, hydroxide, or sulfate anion.In some implementations, such salt is present in the salt solution at aconcentration of about 10 uM to about 200 mM, or about 100 uM to about20 mM.

In some cases, a bipolar MEA is configured to produce carbon monoxide byreducing carbon dioxide at the cathode layer, and the salt ions comprisealkali metal ions. In some cases, the bipolar MEA comprisessubstantially no transition metal ions.

In certain embodiments, all polymers in the MEA are anion conductingpolymers, and the carbon oxide reduction catalyst comprises copper, andwherein the salt comprises (i) an alkali metal cation and (ii) abicarbonate or hydroxide anion. In some implementations, the salt ispresent in the salt solution at a concentration of about 10 mM to about15M, or about 50 mM to about 1M.

In certain embodiments, the MEA with anion conducting polymers isconfigured to produce methane by reducing carbon dioxide and/or carbonmonoxide at the cathode layer, and wherein the salt ions comprise sodiumions. Such MEA may be configured to produce one or more organiccompounds having two or more carbon atoms by reducing carbon dioxideand/or carbon monoxide at the cathode layer, and the salt ions maycomprise ions potassium, cesium, rubidium, or any combination thereof.

Some aspects of the disclosure pertain to electrochemical systemsconfigured to electrolytically reduce a carbon oxide. Such systems maybe characterized by the following features: (a) a membrane electrodeassembly (MEA) comprising: (i) a cathode layer comprising a carbon oxidereduction catalyst that promotes reduction of a carbon oxide, (ii) ananode layer comprising a catalyst that promotes oxidation of a water,and (iii) a polymer electrolyte membrane (PEM) layer disposed between,and in contact with, the cathode layer and the anode layer; and (b) asource of anode water comprising a salt having a concentration of atleast about 10 uM in the anode water, wherein the source of anode wateris connected to the MEA in a manner allowing the anode water to contactthe anode layer and provide the salt to the MEA.

In certain embodiments, the carbon oxide reduction catalyst comprisesgold, silver, copper, or a combination thereof. In certain embodiments,the cathode layer comprises an anion conducting polymer. In certainembodiments, the anode layer comprises a cation conducting polymer.

In some implementations, the PEM layer comprises a polymer electrolytelayer and a cathode buffer layer. As an example, the PEM layer maycomprise a cation conducting polymer and the cathode buffer layercomprises and anion conducting polymer. In some implementations, the PEMlayer comprises an anion conducting polymer.

In certain embodiments, the salt comprises alkali metal ions. In certainembodiments, the salt comprises an anion selected from the groupconsisting of phosphate, sulfate, carbonate, bicarbonate, and hydroxide.

In some cases, the MEA of an electrochemical system is a bipolar MEA,having at least one layer of a cation conducting polymer, and at leastone layer of an anion conducting polymer.

In certain bipolar MEA embodiments, the carbon oxide reduction catalystcomprises copper, and wherein the salt comprises (i) an alkali metalcation, and (ii) a bicarbonate, a sulfate, or a hydroxide anion. As anexample, the salt is present in the anode water at a concentration ofabout 1 mM to about 1M, or about 1 mM to about 50 mM. In someimplementations, the bipolar MEA is configured to produce methane byreducing carbon dioxide and/or carbon monoxide at the cathode layer, andthe salt comprises sodium ions. In some implementations, the bipolar MEAis configured to produce one or more organic compounds having two ormore carbon atoms by reducing carbon dioxide and/or carbon monoxide atthe cathode layer, and the salt comprises ions of potassium, cesium,rubidium, or any combination thereof.

In certain bipolar MEA embodiments, the carbon oxide reduction catalystcomprises gold, and the salt comprises (i) an alkali metal cation and(ii) a bicarbonate, hydroxide, or sulfate anion.

In some cases, the salt is present in the anode water at a concentrationof about 10 uM to about 200 mM, or about 100 uM to about 20 mM. In somecases, the bipolar MEA is configured to produce carbon monoxide byreducing carbon dioxide at the cathode layer, and the salt comprisesalkali metal ions. In some implementations, the bipolar MEA configuredto produce carbon monoxide comprises substantially no transition metalions.

In certain embodiments, all polymers in the MEA are anion conductingpolymers, and the carbon oxide reduction catalyst comprises copper, andwherein the salt comprises (i) an alkali metal cation and (ii) abicarbonate or hydroxide anion. In some implementations, the salt ispresent in the anode water at a concentration of about 10 mM to about15M, or about 50 mM to about 1M. In certain embodiments, the MEA withanion conducting polymers is configured to produce methane by reducingcarbon dioxide and/or carbon monoxide at the cathode layer, and whereinthe salt comprises sodium ions. In certain embodiments, the MEA withanion conducting polymers is configured to produce one or more organiccompounds having two or more carbon atoms by reducing carbon dioxideand/or carbon monoxide at the cathode layer, and wherein the saltcomprises ions potassium, cesium, rubidium, or any combination thereof.

In certain embodiments, electrochemical system additionally includes arecirculation loop connected to the MEA and configured to recover anodewater from the MEA, store and/or treat recovered anode water, and supplystored or treated anode water to the MEA. In some cases, therecirculation loop comprises a reservoir for storing the anode thewater. In some cases, the recirculation loop comprises an inlet forreceiving purified water. In certain embodiments, the recirculation loopcomprises an anode water purification element configured to removeimpurities from the anode water. In some embodiments, the recirculationloop is connected to the source of anode water.

In certain embodiments, the electrochemical system additionally includesa cathode water conduit connected to the anode water recirculation loop.The cathode water conduit may be configured to provide the recirculationloop with water recovered from a carbon oxide stream after the carbonoxide stream has contacted the cathode layer of the MEA. In some cases,the electrochemical system additionally includes a water separatorcoupled to the cathode water conduit and configured to separate cathodewater from the carbon oxide stream.

Other aspects of the disclosure pertain to methods of electrolyticallyreducing a carbon oxide. Such methods may be characterized by thefollowing operation (in any order): (a) providing a salt solution to amembrane electrode assembly (MEA) comprising (a) a cathode layercomprising a carbon oxide reduction catalyst that promotes reduction ofa carbon oxide; (b) an anode layer comprising a catalyst that promotesoxidation of a water; and (c) a polymer electrolyte membrane (PEM) layerdisposed between, and in contact with, the cathode layer and the anodelayer, wherein the salt solution comprises at least about 10 uM of asalt; and (b) electrolytically reducing a carbon oxide at the cathode ofthe MEA while the MEA is in contact with the salt solution.

In various embodiments, the methods may employ MEAs, salts, andassociated system components as set forth above for the MEA andelectrochemical system aspects of this disclosure. Note that while someaspects described above supply salt to an MEA via anode water, not allmethods require this. For example, the salt may preloaded to the MEA byinfusing salt into the MEA prior to operation.

In some embodiments, the methods provide the salt solution to the MEA bysupplying anode water to the anode layer of the MEA. In someimplementations, the methods additionally include (i) recovering anodewater that was supplied to the MEA, and (ii) recirculating recoveredanode water to the anode layer of the MEA. In some implementations, themethods additionally include storing and/or treating the recovered anodewater before recirculating the recovered anode water to the anode layerof the MEA. In some implementations, the methods additionally includepurifying the anode water and/or the recovered anode water to removeimpurities from the anode water.

In certain embodiments, the methods additionally include (i) recoveringwater from a carbon oxide stream after the carbon oxide stream hascontacted the cathode layer of the MEA, and (ii) providing recoveredwater from the carbon oxide stream to the anode layer of the MEA.

These and other features of the disclosure will be presented in moredetail below with reference to the associated drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is an illustration of an example of an electrolytic carbon oxidereduction system that may be used to control water composition and flowin an MEA cell.

FIG. 1B is an illustration of an example of an electrolytic carbon oxidereduction system that may be used to control water composition and flowin an MEA cell.

FIG. 2 is a schematic illustration of a membrane electrode assembly foruse in CO_(x) reduction, according to an embodiment of the disclosure.

FIG. 3 is an illustration of a bipolar MEA in which bicarbonate and/orcarbonate ions may combine with hydrogen ions between the cathode layerand the anode layer to form carbonic acid, which may decompose to formgaseous CO₂.

FIG. 4 is an illustration of an MEA in which CO₂ gas is provided to acathode catalyst layer.

FIG. 5 is an illustration of an MEA having a cathode catalyst layer, ananode catalyst layer, and an anion-conducting PEM configured to promotea CO reduction reaction.

FIG. 6 is a schematic drawing showing an example morphology of cathodeparticles having catalysts supported on a catalyst support particle.

FIG. 7 is an illustration of an MEA similar to that shown FIG. 3, butadditionally shows information relevant to mass transport and generationof CO₂ and water at a bipolar interface.

FIGS. 8A-8D present various MEA designs that contain features thatresist delamination and optionally provide a pathway for the reactionproducts to leave the interface area.

FIG. 9 is an illustration of a partial MEA that includes ananion-conducting polymer layer, which may be a cathode buffer layer, anda polymer electrolyte membrane, which may be cation-conducting polymerlayer.

FIG. 10 is a schematic drawing that shows the major components of aCO_(x) reduction reactor (CRR), according to an embodiment of theinvention.

FIG. 11 is a schematic drawing that shows the major components of a CRRwith arrows showing the flow of molecules, ions, and electrons accordingto one embodiment of the invention.

FIG. 12 is a schematic drawing that shows the major inputs and outputsof the CRR reactor.

FIGS. 13A and 13B presents performance plots for two carbon dioxideelectrolyzers, one with no salt in the anode water and one with 2 mMNaHCO₃ in the anode water.

FIG. 14 presents plots demonstrating that salt has a performanceenhancing effect in Faradaic yield and voltage efficiency of methane andethylene producing CO₂ electrolyzer systems. The cells employed abipolar MEA and a copper catalyst (cathode).

FIG. 15 presents experimental data showing that 6 mM NaHCO₃ showed thehighest Faradaic yield, as compared to concentrations 2 mM, 8 mM, and 10mM. The result is dependent on the size of surface area of the cell. AllMEA cells employed a bipolar MEA and a gold catalyst (cathode).

FIG. 16 shows an example in which salt concentration impacts productionof C2 hydrocarbon (e.g., ethylene and ethanol) yields.

FIG. 17 presents data from an experiment in which anode water waschanged from NaHCO₃ to KHCO₃ during a reaction. The selectivity formethane declined, while the selectivity for ethylene increased.

In FIG. 18 presents data illustrating improved selectivity towardethanol when using KHCO₃ versus NaHCO₃.

FIGS. 19A and 19B (table) illustrate the selectivity and voltageimprovement after fresh salt solution is added or replace the oldsolution in the anolyte reservoir.

FIG. 20 presents a scan of salt concentration versus the selectivity ofa copper catalyst toward methane in the range of 1 mM to 30 mM NaHCO₃.

FIGS. 21A and 21B present data from a test of various salts for effecton ethylene selectivity at different concentrations.

DETAILED DESCRIPTION Introduction and Overview

Polymer-electrolyte membrane electrolyzers are used for electrolyzingwater to produce oxygen at the anode and hydrogen at the cathode. In atypical water electrolyzer, care is taken to prepare themembrane-electrode assembly so that no ions besides H⁺ or OH⁻ areintroduced. And, during operation, only pure water is introduced to theanode side of the cell.

An electrolyzer system of the present disclosure can produce oxygen atthe anode from water oxidation and one or more carbon-based compoundsthrough the electrochemical reduction of carbon dioxide or other carbonoxide introduced to the cathode. As used herein, the term carbon oxideincludes carbon dioxide and/or carbon monoxide. In some embodiments,carbon monoxide is used as a reducible reactant. In some embodimentscarbon dioxide is used as a reducible reactant. In some embodiments, amixture of carbon dioxide and carbon monoxide is used as a reduciblereactant.

In contrast to water electrolyzers, where salt ions are not desirable,the inventors have found that salt ions can have a positive impact oncarbon oxide electrolyzer performance. Cations may be introduced to thecarbon oxide electrolyzer through water circulating through the anode ofthe electrolyzer or by incorporation into the polymer-electrolytemembrane, catalyst, or catalyst support used to make themembrane-electrode assembly.

The presence of salts has been observed to decrease the MEA cellvoltage, improve Faradaic yield, change the product selectivity, and/ordecrease the decay rate of operating parameters (e.g., voltageefficiency) during operation of a carbon oxide reduction electrolyzer.

The introduction of salt ions may affect the carbon oxide electrolysisperformance through any of several possible mechanisms. While notwishing to be bound by theory, the following is list of examplemechanisms by which salts may influence operation of an MEA cell duringelectrolytic carbon oxide reduction.

The presence of cations and/or anions from a salt reduces the activationenergy of one or more catalytic pathways. This may be due to any of manypossible mechanisms. For example, a salt may change the localelectrolyte structure and/or electron density on the catalyst surface.It has been observed that salt ions increase in Faradaic yield in somecarbon oxide reduction systems. It has also been observed that thepresence of particular ions changes the selectivity of a catalyst forone reaction over another.

Cations and/or anions from a salt may help hydrate polymer-electrolyte,particularly anion exchange polymers. Ions travel as hydrates; i.e.,they carry water molecules with them as they move across polymer layers.Hydration of the MEA, and particularly portions the MEA close to thecathode catalyst, may facilitate the reduction reaction by preventingthe flowing carbon oxide from evaporating water in the MEA. In general,salt ions may promote hydration of the MEA, particularly at regions ofthe MEA susceptible to drying. In various embodiments, the presence ofsalt in the polymer renders the polymer more hygroscopic.

The presence of salts and the ions from a salt may increase theconductivity of one or more MEA layers. In particular, the ions mayincrease the conductivity of anion exchange polymers, which tend to haverelatively low conductivity compared to cation exchange polymers.Increasing conductivity of the polymers may reduce the overallresistance of the MEA cell.

The presence of a salt may raise the pH of one or morepolymer-electrolyte layers. This should be compared with proton donatingadditives, which lower the polymers' pH.

The presence of cations and/or anions from a salt changes water uptakeand swelling of polymer electrolyte layers. If volumetric changes due toswelling are mismatched between the anode side and the cathode side ofan MEA, mechanical stress on the MEA can degrade cell performance. Incertain embodiments, the presence of a salt at a defined concentrationtunes the relative amounts of swelling in two or more different layersof an MEA to equalize the swelling exhibited by these layers.

The presence of cations and/or anions provided by salts may change theconductivity at the interface between two layers of the MEA. In abipolar interface, for example, protons may have to jump across aninterfacial gap to meet anions. This jump has an associated resistance.The presence of a salt may decrease the barrier to protons and anionscoming together across the interface. Note that pores in Nafion andsimilar polymers have sulfonic acid groups to allow protons to move withlow resistance. At a bipolar interface, these groups are not present tofacilitate continued movement. A salt can provide a non-charge depletedregion at the interface to facilitate protons and anions coming together(e.g., protons come from the anode side and react with bicarbonate ionsfrom the cathode side). Stated another way, a salt solution present atthe interface may provide a conductive bridge or and ionicallyconductive bridge between the anion conducting polymer and the cationconducting polymer.

Cations and/or anions provided by a salt may provide a counter ion forcharged carbon-based species formed by the cathode reduction reaction.Such charged species require an available counter ion to maintain chargeneutrality. In some implementations, the reduction reaction at thecathode produces a carboxylate product (e.g., formate, oxalate, oracetate). However, if there are relatively few available cations, thereaction may be disfavored. This may be the case where the cathode layercomprises an anion exchange polymer, such as an anion exchange membrane(AEM), which blocks the flux of protons (potential counterions). Cationsdonated by a salt may provide the needed species to facilitatecarboxylate-producing reactions.

A salt concentration gradient may induce osmotic pressure. For example,the salt concentration may be greater on the anode side, which drawswater away from the cathode and thereby reduces the occurrence ofcathode flooding. Note that water present on the cathode side may beprovided, at least in part, by reaction of hydrogen ions and bicarbonateions in the MEA interior. This water does not initially have salt ions,which contributes to the concentration gradient.

Characteristics of Salt Used in MEA Cell

Various types of salt may be used in an MEA cell. Such salts may haveinorganic or organic cations and anions. The salt composition may affectcell operating conditions such as overpotential, Faradaic efficiency,and/or selectivity among multiple carbon oxide reduction reactions.Various factors influencing the choice of salt composition are describedherein.

Cation Reactivity

The salt composition may depend on the catalyst used at the cathode. Incertain embodiments, the salt does not contain a cation that couldpoison the cathode catalyst. For example, the salt may not contain acation that could be reduced at a cathode catalyst such as a catalystcomprising gold or another noble metal. Such catalysts are sometimesused in MEA cells configured to reduce carbon dioxide to carbon monoxideor other reduction product. It has been found that reduction of metalions such as iron or other transition metal ions on catalyst particlescan poison the catalyst or otherwise decrease the catalytic conversionof carbon dioxide to a reduction product such as carbon monoxide.

In certain embodiments, a salt employed in a carbon oxide reductionreactor contains only cations that are not reducible in an aqueousmedium to elemental metal under operating conditions for carbon dioxidereduction at a cathode. In certain embodiments, a salt employed in thereactor does not have transition metal ions. In certain embodiments, asalt employed in the reactor has only alkali metal cations and/oralkaline earth element cations.

While generation of carbon monoxide from carbon dioxide may be performedwith a gold or silver catalyst, generation of hydrocarbons and/ororganic oxygen-containing compounds from a carbon oxide may be performedwith a copper or other transition metal catalyst at the cathode. In somecases, a salt employed in a cell configured to produce hydrocarbonsand/or organic oxygen-containing compounds has one or more cations thatare not alkali metal ions or alkaline earth element ions. For example,an MEA with a transition metal catalyst may be configured with a saltcomprising one or more transition metals.

The types of salts used as well as their concentration may varydepending upon whether the carbon oxide reduction reactor is one thatuses a bipolar MEA, one that uses an anion exchange polymer only MEA, orone that uses some other MEA configuration. A cell configured to reducecarbon monoxide may employ an anion exchange polymer only MEA becauselittle or no bicarbonate is formed at the cathode and so the MEA neednot include a cation-conducting polymer to block bicarbonate transportto the anode where it could liberate carbon dioxide that would otherwisebe used in a reduction reaction at the cathode. Such cells may employsalts that contain cations of transition metals or other metals thatmight poison a noble metal catalyst. In certain embodiments, a carbondioxide reduction cell having a bipolar MEA employs a salt that does nothave transition metal ions.

In certain embodiments, the salt contains a cation that adjusts the pHat one or more locations in a carbon oxide reducing cell (e.g., at theanode, the cathode, or an intermediate ionically conductive polymerlayer). In some cases, during operation, the salt adjusts the pH to bemore acidic or more basic at one or more such locations. In certainembodiments, the anion is ammonium, a derivatized ammonium cation suchas a quaternary ammonium ion, an alkali metal ion, or an alkaline earthmetal ion.

Anion Reactivity

The salt composition may be influenced by the reaction at the anode of acarbon dioxide reduction cell. In certain embodiments, a salt containsan anion that does not readily oxidize at the anode and/or does notreadily reduce at the cathode under operating conditions of the cell. Incertain embodiments, the anion is not a halide. In some cases, the anionis not chloride, bromide, or iodide. Halides potentially oxidize at theanode where they could form elemental halogen. Note that in certainembodiments, however, a halide is used in a carbon dioxide reductioncell where the reduction product is a halogenated compound. In certainembodiments, a salt has an anion that is not an oxidizablenitrogen-containing anion such as a nitrite or an amine. In certainembodiments, a salt has an anion that is not an organic anion; forexample, the salt does not contain a carboxylate ion.

In certain embodiments, the salt contains an anion that adjusts the pHat one or more locations in a carbon oxide reducing cell (e.g., at theanode, the cathode, or an intermediate ionically conductive polymerlayer). In some cases, during operation, the salt adjusts the pH to bemore acidic or more basic at one or more such locations. In certainembodiments, the anion is hydroxide, bicarbonate, sulfite, or sulfate.

Ionic Mobility

One consideration in choosing the cation and/or an anion of a salt isthe ion's mobility. In certain embodiments, the ion has a relativelyhigh mobility in the polymers of an MEA. In some cases, one or morelayers of an MEA with the salt present each have an ionic conductivityof at least about 4 mS/cm. In some implementations, ions that arerelatively small in atomic weight are used. In some cases, the cation ofthe salt has an atomic or molecular weight of about 140 or lower, orabout 90 or lower, or about 40 or lower. In some cases, the anion of thesalt has an atomic or molecular weight of about 100 or lower.

Solubility

In certain embodiments, the salt is relatively soluble in aqueous media.For example, the salt may have a solubility of at least about 1 mol/L,or least about 2 mol/L, or at least about 10 mol/L in otherwisedeionized water at 25° C.

Product Selectivity, Voltage Efficiency, Lifetime Improvement, and DecayRate Decrease

The type of the salt can impact product selectivity in an MEA cell. Thechoice of one cation over another may change the ratio of one productover another by, e.g., at least about 10%.

In certain embodiments, a sodium-containing salt such as sodiumbicarbonate when used in an MEA cell with a gold catalyst on the cathodeselectively increases production of carbon monoxide over the byproducthydrogen during carbon dioxide reduction. This increase in carbonmonoxide production is observed in comparison to similar goldcatalyst-containing MEA cells containing no salt. For example, an MEAcell having a gold catalyst and employing sodium bicarbonate mayincrease the carbon monoxide production by at least about 100% whencompared to a similar MEA cell that uses no salt. In other words, an MEAcell employing a sodium-containing salt such as sodium bicarbonategenerates carbon monoxide in a molar quantity that is at least abouttwo-fold higher than that produced by the same MEA cell operated in thesame way but with substantially no salt. In some embodiments, the MEAcell employing a sodium containing salt generates carbon monoxide in amolar quantity that is at least about three-fold higher. In some cases,an MEA cell employing a potassium containing salt such as potassiumbicarbonate generates carbon monoxide in a molar quantity that is atleast about two-fold higher than that produced by the same MEA celloperated in the same way but with substantially no salt. In some cases,an MEA cell employing a salt with higher atomic weight alkali metal suchas cesium or rubidium generates carbon monoxide in a molar quantity thatis at least about two-fold higher than that produced by the same MEAcell operated in the same way but with substantially no salt.

In some implementations, an MEA cell configured to produce carbonmonoxide from carbon dioxide employs an alkali metal containing salt andis operated in a manner that produces products at the cathode having atleast about 70 mole % carbon monoxide or at least about 80 mole % carbonmonoxide among various products, but not including unreacted carbondioxide. Other products that may be produced at the cathode includehydrogen, one or more hydrocarbons, one or more carbonyl-containingproducts, and the like. An MEA cell configured to produce carbonmonoxide may comprise gold, silver, or other noble metal at the cathode.An MEA cell configured to produce carbon monoxide may comprise a bipolarmembrane assembly.

In certain embodiments, the concentration of a sodium, potassium,cesium, or rubidium containing salt in water delivered to an MEA cell isabout 1 mM to 20 mM. This concentration range may apply to MEA cellsconfigured to produce carbon monoxide from carbon dioxide. In certainembodiments, such cells comprise a gold or other noble metal as acathode catalyst. As used herein, a noble metal is a metal that stronglyresists chemical action. Examples include platinum and silver, inaddition to gold.

In some cases, MEA cells with relatively smaller surface areas (e.g.,about 10 cm² to about 50 cm² assuming a planar face) skew to arelatively lower concentration range, such as from about 1 mM to 5 mM,while MEA cells with relatively larger surface areas (e.g., about 50 cm²to about 1000 cm²) skew to a relatively higher concentration range, suchas from about 5 mM to 20 mM.

In certain embodiments, a salt such as sodium bicarbonate when suppliedvia anode water to an MEA cell with a gold catalyst on the cathodeimproves energy efficiency by about 9%-25%. Further, in certainembodiments, desirable levels of selectivity for CO and cell voltage,observed during initial operation of a cell, are more than an order ofmagnitude more stable when the salt solution is used.

In some cases, an MEA cell employing a sodium containing salt such assodium bicarbonate has a voltage efficiency for producing carbonmonoxide that is at least about 5% higher than the voltage efficiency ofthe same MEA cell operated in the same way but with substantially nosalt. In some cases, the MEA cell employing a sodium containing saltsuch as sodium bicarbonate has a voltage efficiency for producing carbonmonoxide that is at least about 10% higher, or at least about 20%higher, than the voltage efficiency of the same MEA cell operated in thesame way but with no substantially salt. In some cases, an MEA cellemploying a potassium containing salt such as potassium bicarbonate hasa voltage efficiency for producing carbon monoxide that is at leastabout 5% higher than the voltage efficiency of the same MEA celloperated in the same way but with substantially no salt. In some cases,an MEA cell employing a salt with a higher atomic weight alkali metalsuch as cesium or rubidium has a voltage efficiency for producing carbonmonoxide that is at least about 5% higher than the voltage efficiency ofthe same MEA cell operated in the same way but with substantially nosalt. In certain embodiments, the voltage efficiency for producingcarbon monoxide in a bipolar MEA having gold or other noble metalcathode catalyst is at least about 25%.

As an example, a tested cell with no salt in the anode water has anaverage voltage of 3.86V and an average CO Faradaic yield of 0.53 forthe first hour at 0.5 A/cm² and decay rate of 144 mV/hour and 0.018 COFaradaic yield/hour for hours 2-5 at 500 mA/cm². In comparison, the samecell operated with 2 mM NaHCO₃ has an average voltage of 3.52 V and anaverage CO Faradaic yield of 0.936 for the first hour at 0.5 A/cm² anddecay rate of 15.5 mV/hour and 0.001 CO Faradaic yield/hour for hours2-5 at 500 mA/cm². In certain embodiments, an MEA cell configured toproduce carbon monoxide from carbon dioxide has an average voltage of atmost about 3.6 V for the first hour of operation and a decay rate of nomore than about 16 mV/hour for hours 2-5.

In certain embodiments, the voltage efficiency and/or the productselectivity for carbon monoxide production in an MEA cell employing asodium, potassium, cesium, or rubidium containing salt in water isstable over a period of operation that is at least 10 times longer thanthat of a corresponding MEA cell operated in the same way, over the sameperiod, but with substantially no salt. In certain embodiments, thevoltage for carbon monoxide production in an MEA cell employing anaqueous sodium, potassium, cesium, or rubidium containing salt does notincrease by more than about 0.5%, or by more than about 16 mV per hour,at an applied current density of 600 mA/cm² or lower for more than 8hours of operation. In certain embodiments, the mole fraction of carbonmonoxide among all other products (excluding carbon dioxide) produced atthe cathode of an MEA cell employing an aqueous sodium, potassium,cesium, or rubidium containing salt does not decrease by more than about1% per hour, at an applied current density of 600 mA/cm² or lower formore than 8 hours of operation. In certain embodiments, the voltage forcarbon monoxide production in an MEA cell employing an aqueous sodium,potassium, cesium, or rubidium containing salt does not increase by morethan about 0.03%, or by more than about 0.05 mV per hour, at an appliedcurrent density of 300 mA/cm² or below for more than 100-hour operation.In certain embodiments, the mole fraction of carbon monoxide among allother chemicals produced at the cathode of an MEA cell employing anaqueous sodium, potassium, cesium, or rubidium containing salt does notdecrease by more than about 0.1% per hour, at an applied current densityof 300 mA/cm² or lower for more than 100-hour operation.

Faraday efficiency, which is also sometimes referred to as Faradaicyield, coulombic efficiency or current efficiency, is the efficiencywith which charge is transferred in a system facilitating anelectrochemical reaction. The use of Faraday's constant in Faradaicefficiency correlates charge with moles of matter and electrons.Faradaic losses are experienced when electrons or ions participate inunwanted side reactions. These losses appear as heat and/or chemicalbyproducts.

Voltage efficiency describes the fraction of energy lost throughoverpotential or resistance to charge movement in the MEA cell. For anelectrolytic cell this is the ratio of a cell's thermodynamic potentialdivided by the cell's experimental cell voltage, converted to apercentile. Losses in a cell's voltage due to overpotentials aredescribed by voltage efficiency. For a given type of electrolysisreaction, electrolytic cells with relatively higher voltage efficiencieshave relatively lower overall cell voltage losses due to resistance.

In certain embodiments, a sodium containing salt such as sodiumbicarbonate when used in a bipolar MEA cell with copper catalyst on thecathode produces methane with improved voltage efficiency in proportionwith increasing salt concentration. An increase in voltage efficiency by6.5% was observed when increasing salt concentration from 3 mM to 20 mMsodium bicarbonate.

In certain embodiments, a sodium-containing salt such as sodiumbicarbonate when used in a bipolar MEA cell with copper catalyst on thecathode produces methane with improved voltage efficiency as compared todeionized water. At least about a 30% improvement in initial voltageefficiency and at least about 8× improvement in voltage decay rate wasseen when sodium bicarbonate was used as anolyte as compared todeionized water.

In certain embodiments, a potassium containing salt such as potassiumbicarbonate used in an MEA cell having a copper catalyst on a cathodeselectively produces ethanol and ethylene over methane during carbondioxide reduction. By contrast, a sodium containing salt such as sodiumbicarbonate when used in an MEA cell having a copper catalyst on acathode selectively produces methane during carbon dioxide reduction. InMEA cells employing copper reduction catalysts, salts with higher atomicweight cations increase the Faradaic yield of multi-carbon products(e.g., ethylene).

In one example, a bipolar MEA setup with copper catalyst at the cathodewas used with anolyte sodium bicarbonate in the concentration of 3 mM togive product selectivity distribution of about 61.3 mole % methane,about 3 mole % ethylene, about 1.3 mole % carbon monoxide and about 28.5mole % hydrogen. This demonstrated a high ratio of methane to ethylene(over 20:1) when carbon dioxide electrolysis is performed in thepresence of a sodium salt.

In one example, a cell comprising a bipolar MEA with copper catalyst atthe cathode and sodium bicarbonate salt as anolyte at a conductivity of279 microSiemens (˜3 mM concentration) was shown to produce about 40%methane, about 20 mole % ethylene, about 1 mole % carbon monoxide andabout 17 mole % hydrogen. When the salt solution of the same setup waschanged to potassium bicarbonate of a similar conductivity (˜2 mM) asignificant product selectivity change was observed. The total ethyleneand liquid C2-C3 production was increased by about 170 mole %,accompanied by about a 73 mole % decrease in methane production, andabout a 40 mole % decrease in hydrogen.

In various embodiments, potassium cation salts favor a selectivity forethylene by a mole ratio of at least about 5:1 over methane. In variousembodiments, sodium cation salts favor a selectivity of methane by amole ratio of at least about 20:1 over ethylene. These embodiments applyto bipolar MEA cells. In some cases, the MEA cells employ a coppercatalyst. Cesium has a similar effect as potassium with bipolar MEAcells.

In certain embodiments, a bipolar MEA cell with copper catalyst and asodium-containing salt gives a methane from carbon dioxide Faradaicefficiency of at least about 50% (e.g., up to about 73%). In certainembodiments, a bipolar MEA cell with copper catalyst and apotassium-containing salt gives an ethylene from carbon dioxide Faradaicefficiency of at least about 20% (e.g., up to about 33%). Cesium may beemployed with similar effect to potassium with bipolar MEA cells. Incertain embodiments, an anion conducting polymer only cell with coppercatalyst and a potassium-containing salt gives an ethylene from carbondioxide Faradaic efficiency of at least about 30% (e.g., about 41%).

In some implementations, an MEA cell configured to produce methane fromcarbon dioxide employs a sodium containing salt and is operated in amanner that produces products at the cathode having at least about 50mole % methane or at least about 70 mole % methane. Other products thatmay be produced at the cathode include hydrogen, carbon monoxide, one ormore two or more carbon organic molecules, and the like. An MEA cellconfigured to produce methane may comprise copper or other transitionmetal at the cathode. An MEA cell configured to produce methane maycomprise a bipolar membrane assembly.

In some implementations, an MEA cell configured to produce ethyleneand/or other organic compounds having two or more carbon atoms fromcarbon dioxide employs a potassium, cesium, or rubidium-containing saltand is operated in a manner that produces products at the cathode havingat least about 60 mole % ethylene and/or other organic compounds havingtwo or more carbon atoms or at least about 80 mole % ethylene and/orother organic compounds having two or more carbon atoms. Other productsthat may be produced at the cathode include hydrogen, methane, andcarbon monoxide. An MEA cell configured to produce ethylene and/or otherorganic compounds having two or more carbon atoms may comprise copper orother transition metal at the cathode. An MEA cell configured to produceethylene and/or other organic compounds having two or more carbon atomsmay comprise a bipolar membrane assembly.

In certain embodiments, the voltage efficiency and/or the productselectivity for methane or organic compound production in an MEA cellemploying a sodium, potassium, cesium, or rubidium containing salt inwater does not decrease by more than about 1%, or by more than about0.3%, or by more than about 0.01%, over 90 A-hr.

The cathode catalysts described herein include alloys, doped materials,and other variants of the listed material. For example, an MEA cathodecatalyst described as containing gold or other noble metal is understoodto include alloys, doped metals, and other variants of gold or othernoble metals. Similarly, an MEA cathode catalyst described as containingcopper or other transition metal is understood to include alloys, dopedmetals, and other variants of copper or other transition metals.

Representative Examples of Salts

In certain embodiments, a salt employed in the reactor has cations thatare not ions of transition metals. In certain embodiments, the saltcontains a cation that is an alkali metal on or an alkaline earth metalion. In certain embodiments, the salt contains a lithium ion, sodiumion, potassium ion, cesium ion, and/or a rubidium ion. In certainembodiments, the salt contains no cations other than sodium, and/orpotassium ions. In some implementations, the salt contains only cationsthat are monovalent such as alkali metal ions.

In certain embodiments, the salt contains an anion that is hydroxide,bicarbonate, carbonate, perchlorate, phosphate, or sulfate. In somecases, the salt contains an anion that is hydroxide, bicarbonate,carbonate, or sulfate. In certain embodiments, the salt contains nohalide ions. In certain embodiments, the salt contains an anion that isproduced from the carbon oxide reduction reaction. Examples includecarboxylates such as formate, oxalate, and acetate.

In certain embodiments, the salt is selected from the group includingsodium bicarbonate, potassium bicarbonate, potassium sulfate, sodiumsulfate, cesium bicarbonate, cesium sulfate, and any combinationthereof.

In some cases, an MEA employs multiple salts or a mixed salt. Forexample, the MEA may employ multiple cations (e.g., sodium and potassiumions) but only a single anion (e.g., sulfate). In another example, theMEA employs only a single cation (e.g., sodium ions) but multiple anions(e.g., bicarbonate and sulfate). In yet another example, the MEA employsat least two cations and at least two anions. In certain embodiments,the salts include a combination of sodium bicarbonate and potassiumbicarbonate. In certain embodiments, the salts include a combination ofpotassium bicarbonate and potassium phosphate.

Delivery of Salt to MEA

A salt may be delivered to the cell in various ways. In one example, asalt is provided with an MEA as fabricated and/or is provided with areconstituted MEA. In another example, a salt is provided with afeedstock (a reactant containing composition) to the anode or cathode.In some implementations, water is a reactant at the anode and a salt isprovided with the anode reactant. Water supplied to the anode issometimes termed “anode water.” The anode water may be an aqueoussolution that, during operation, is flowed to the anode. In someembodiments, the anode reaction is oxidation of water to produce oxygen.In some embodiments, liquid water containing a salt is delivered to thecathode in any of various ways. For example, the salt may be deliveredvia flowing a liquid solution to the cathode during operation. Theliquid may contain dissolved carbon dioxide or dissolved carbonmonoxide. In some cases, aqueous solutions of salt are delivered to thecathode as a mixture of liquid and gas. For example, a salt solution maybe sprayed on the MEA.

Salt-containing solution provided to the MEA directly or via anode waterduring operation may be prepared in various ways. In some cases,salt-containing solutions are made by dissolving salt directly in water.In some cases, salt-containing solutions are made by passing waterthrough a resin (optionally in a column) that releases salt into thewater.

Salt Concentration

In embodiments where salt is provided to the MEA by way of liquid watersuch as anode water, the salt may be provided at a set concentration.The salt concentration may vary depending upon the MEA configuration andthe particular cathode catalyst employed, as well as the associatedcarbon oxide reduction reaction.

In some embodiments employing a bipolar membrane MEA, the salt isprovided in an aqueous solution at a concentration of about 1 mM toabout 30 mM or at a concentration of about 3 mM to about 30 mM. In someembodiments employing a bipolar membrane MEA, the salt is provided at aconcentration of about 2 mM to about 15 mM. In some embodimentsemploying a bipolar membrane MEA, the salt is provided at aconcentration of about 0.1 mM to about 30 mM, or about 5 mM to about 10mM.

In some embodiments employing a bipolar membrane MEA configured forhydrocarbon production from carbon dioxide, the salt is provided inanode water or other source at a concentration of about 2 mM to about 50mM. In some MEAs employed in cells configured for methane productionfrom carbon dioxide, the salt is provided in a concentration of about 10mM to 30 mM. In various implementations, such cells employ a coppercatalyst and a salt selected from the group including sodiumbicarbonate, potassium bicarbonate, potassium sulfate, sodium sulfate,cesium bicarbonate, cesium sulfate, and any combination thereof. Invarious embodiments, the salt employed for methane selectivity is sodiumbicarbonate, which has been shown to enhance methane to ethylene ratioby at least about 20:1.

In certain embodiments employing a bipolar membrane MEA configured forhydrocarbon product generation from a carbon oxide, and particularlycarbon dioxide, the salt is provided at a concentration of about 2 mM to1 M. In some implementations, the salt is potassium bicarbonate, whichhas been shown to enhance C2-C3 product selectivity over methane by aratio of about 5:1 compared to sodium bicarbonate, is provided at aconcentration of about 100 mM to about 500 mM. In certain embodiments,where the MEA is configured with a copper catalyst as cathode to reducecarbon dioxide to ethylene, the potassium bicarbonate concentration isabout 1 mM to 5 mM. In certain embodiments, where the MEA is configuredto reduce carbon monoxide to ethylene, the salt concentration,particularly potassium bicarbonate, is about 150 mM to about 250 mM.

In some embodiments employing an MEA containing only anion-conductingpolymer(s), the salt is provided in an aqueous solution at aconcentration of about 1 mM to 10 molar. In some embodiments employingan MEA containing only anion-conducting polymer, the salt is provided ina concentration of about 100 mM to 5 molar. In certain embodimentsemploying potassium hydroxide as a salt, the salt concentration is about50 to 150 mM. In certain embodiments employing potassium bicarbonate asa salt, the salt concentration is about 4 to 10 mM.

The following concentration ranges are useful for anion conductingpolymer only and bipolar cells employing anode water with potassiumhydroxide and/or potassium bicarbonate. In certain MEA cells employingpotassium hydroxide, the salt concentration is about 10 mM to 15 M. Insome MEA cells employing potassium hydroxide, the salt concentration isabout 50 to 500 mM. In some MEA cells employing potassium hydroxide, thesalt concentration is about 0.5M to −15M. In certain MEA cells employingpotassium bicarbonate, the salt concentration is about 1 mM to 1M. Insome MEA cells employing potassium bicarbonate, the salt concentrationis about 1 to 50 mM. In some MEA cells employing potassium bicarbonate,the salt concentration is about 100 mM to 500 mM.

The following salt concentration ranges are used, in certainembodiments, employing carbon dioxide as a reactant in an MEA cell:

Bipolar membrane for carbon monoxide production (e.g., gold-containingcatalyst): The salt concentration in anode water is about 10 uM-200 mM,or about 100 um to 20 mM, or about 1 mM-10 mM, or about 1 mM-5 mM, orabout 2 mM-5 mM. In certain embodiments, any of these concentrationranges is used when the salt is sodium bicarbonate. In certainembodiments, any of these concentration ranges is used for MEA cellshaving cathode surface areas of about 25 cm².

Bipolar membrane for methane production (e.g., copper-containingcatalyst): The salt concentration in anode water is about 1 mM-40 mM, orabout 10 mM-30 mM, or about 3 mM-20 mM. In certain embodiments, any ofthese concentration ranges is used when the salt is sodium bicarbonate.In certain embodiments, any of these concentration ranges is used forMEA cells having cathode surface areas of about 25 cm².

Bipolar membrane for ethylene production (e.g., copper-containingcatalyst): The salt concentration in anode water is about 100 um to 20mM, or about 1 mM-10 mM, or about 1 mM-5 mM, or about 2 mM-5 mM. Incertain embodiments, any of these concentration ranges is used when thesalt is potassium bicarbonate. In certain embodiments, any of theseconcentration ranges is used for MEA cells having cathode surface areasof about 25 cm².

Anion conducting polymer only MEA for ethylene production (e.g.,copper-containing catalyst): The salt concentration in anode water isabout 0.05M-5M, or about 0.05M-1M, or about 0.5M-1M, or about0.05M-0.5M. In certain embodiments, any of these concentration ranges isused when the salt is potassium hydroxide. In certain embodiments, anyof these concentration ranges is used for MEA cells having cathodesurface areas of about 25 cm².

The following salt concentration ranges are used, in certainembodiments, employing carbon monoxide as a reactant in an MEA cell:

Anion conducting polymer only MEA for ethylene production (e.g.,copper-containing catalyst): The salt concentration in anode water isabout 0.05M-5M, or about 0.05M-1M, or about 0.5M-1M, or about0.05M-0.5M, or about 0.5M-10M. In certain embodiments, any of theseconcentration ranges is used when the salt is potassium hydroxide. Incertain embodiments, any of these concentration ranges is used for MEAcells having cathode surface areas of about 25 cm².

Anion conducting polymer only MEA for methane production (e.g.,copper-containing catalyst): The salt concentration in anode water isabout 0.05M-10M, or about 0.05M-1M, or about 0.05M-0.5M, or about0.5M-10M or about 0.5M-1M. In certain embodiments, any of theseconcentration ranges is used when the salt is potassium hydroxide orsodium hydroxide. In certain embodiments, any of these concentrationranges is used for MEA cells having cathode surface areas of about 25cm².

Bipolar MEA for ethylene production (e.g., copper-containing catalyst):The salt concentration in anode water is about 20 mM-2M, or about 50mM-500 mM, or about 50 mM-250 mM, or about 100 mM-500 mM. In certainembodiments, any of these concentration ranges is used when the salt ispotassium bicarbonate. In certain embodiments, any of theseconcentration ranges is used for MEA cells having cathode surface areasof about 25 cm².

While the salt concentrations provided herein may be appropriate forMEAs of any size, in certain embodiments, they are appropriate for cellsemploying MEAs having a surface area of about 25 cm² and the listedranges may be scaled for cells with MEAs having larger surface areas.For example, in some embodiments, the salt concentrations increase withMEA area increases by a ratio of about 3:4. So, for example, if a saltconcentration of 2 mM is appropriate for a cell having an MEA area of 25cm², the concentration may be increased to 6 mM for a cell having an MEAarea of 100 cm². As used herein, the area of an MEA is the area of ageometric plane at the MEA surface; it does account for pores or otherdeviations from planarity at the MEA surface.

In certain embodiments, the concentration of salt in an MEA, in moles ofsalt per mass of polymer electrolyte, is between about 1 and 3 mM/g. Incertain embodiments, the concentration of salt in the polymer isestimate using conductivity measurements.

In some implementations, the concentration of any impurity other thanintroduced salt in anode or cathode water is very low; e.g., on theorder of parts per million. This is particularly true of anions that areoxidizable at the anode and cations that are reducible at the cathode.In certain embodiments, the water containing one or more introducedsalts has substantially no other ions other than those of the salt. Forexample, the water may contain no more than about 100 ppb of anytransition metal ion other than any transition metal in the introducedsalt. In some cases, the concentration of reducible transition metal ionis no greater than 10 ppb, or no greater than 1 ppb, or no greater than0.1 ppb. In another example, the water contains no more than about 10ppm of any halide ion. In another example, the water contains no morethan about 10 ppm of any cation other than alkali metal ions and/oralkaline earth metal ions. In another example, the water contains nomore than about 10 ppm of any cation other than alkali metal ions. Incertain embodiments, the salt-containing water contains no more thanabout 100 ppm of unintentionally provided ion. In some cases, thesalt-containing water contains no more than about 10 ppm ofunintentionally provided ion, or no more than about 1 ppm ofunintentionally provided ion, or no more than about 0.1 ppm ofunintentionally provided ion.

In certain embodiments, unwanted ions and/or other impurities areremoved from water prior to delivery of the water to a carbon dioxidereducing cell. This may be accomplished by purifying water upstream ofthe anode and/or cathode to which it is delivered. The water may bepurified by any of various techniques such as passing the water througha resin column containing a chelating-type resin such as CR11 availablefrom Sigma-Aldrich. Examples of techniques to achieve ultra-high puritywater include gross filtration for large particulates, carbonfiltration, water softening, reverse osmosis, exposure to ultraviolet(UV) light for TOC and/or bacterial static control, polishing usingeither ion exchange resins or electrodeionization (EDI) and filtrationor ultrafiltration. The specific steps are affected by the startingquality of the water. With certain combinations of steps, it is possibleto purify water to the point where it has a resistance of greater thanabout 18 MOhms. In certain embodiments, a resistance of only about 10MOhm prior to the deliberate addition of salt is sufficient waterpurification for CO₂ electrolysis.

The salt concentration values presented herein may define saltconcentration in an aqueous solution supplied to an MEA cell. Suchsolutions include anode water supplied during cell operation, a solutionin which an MEA is soaked or otherwise contacted to infuse salt, and thelike. The salt concentration may be different in an MEA than in asolution that supplies salt to the MEA. Typically, salt ions willpenetrate the MEA from the solution and then move through the MEA viaone or more transport mechanisms. In one mechanism, salt ions pass intothe MEA via the supply solution. This may be the case when the solutionpermeates the MEA via pores or other openings in the MEA. Once in theMEA, the solution may move under a pressure gradient. The movingsolution carries the salt ions along with it. While the salt ions arecarried in the supply solution, their overall concentration in the MEAmay be reduced because they occupy a greater volume: they occupy thevolume of the supply solution in addition to the volume of the MEApolymers.

Salt ions in the solution may move independently of the bulk solutionunder the influence of a salt concentration gradient (diffusion orosmosis) or under the influence of an electric field (migration). Thesetransport phenomena may also modify the salt concentration within theMEA. Independently of movement within the supply solution, salt ions maymove by ionic conduction through the conductive polymers of the MEA. Forexample, salt cations may move by ionic conduction in the polymer matrixof a cation exchange membrane such as a sulfonated tetrafluoroethylene.And salt anions may move by ionic conduction through the matrix of ananion exchange membrane. The movement of salt ions in such polymermatrixes is sometimes referred to hopping, with the salt ions hoppingbetween adjacent charged sites within a polymer matrix. During operationof an MEA cell, the salt ions within the polymer matrixes have their ownconcentrations that contribute to the overall salt or salt ionconcentration in the MEA.

Due to the above factors and possibly other factors, the saltconcentration in the MEA may be different from the salt concentration inthe supply solution. While the salt concentration values presentedherein typically represent the salt concentrations within the supplysolution, before it penetrates the MEA, the values may also representthe concentration within an MEA. To the extent that the values representconcentrations within an MEA, they should be considered average valuesthroughout the MEA. Note that salt ions may have different molarconcentrations than their source salts. For example, a 1M solution ofsodium sulfate may, when fully dissociated, be viewed as providing a 2Msolution of sodium ions.

Delivery of Salt Via the MEA

In certain embodiments, salt is provided, at least in part, viapre-operation introduction to one or more components of the MEA. Forexample, the PEM, cathode buffer layer, anode buffer layer, anodecatalyst layer, cathode catalyst layer, or any combination thereof maybe pre-loaded with salt. The pre-loading may be performed before,during, or after assembly of individual MEA layers into an MEA stack. Insome implementations, before the assembly, the pre-loading is achievedby soaking different layers of MEA in salt-containing solutions atvarious preferred concentrations. In some implementations, during theassembly, the pre-loading is achieved by adding droplets ofsalt-containing solutions onto different layers of MEAs. In someimplementations, after the assembly, the pre-loading is achieved bycirculating salt-containing solutions at the anode and/or the cathodecompartment.

In certain embodiments, salt is introduced to an MEA after the MEA hasoperated for a time in a carbon oxide reduction cell. In some cases,after a certain amount of usage, the MEA is taken out of service andexposed to a composition that introduces salt into the polymers of theMEA. This may be accomplished, for example, by adding salt to the anodewater or by putting salt-containing water through the cathode of thecell.

Remove Salt from MEA Cell

In certain embodiments, salts can precipitate or otherwise come out ofsolution and accumulate in certain locations within the cell. Forexample, salts may deposit in a cell's flow field and/or MEA layers andultimately foul the cell.

To address this concern or for other reasons, the cell may beperiodically taken off line and exposed to a flow of water (e.g.,deionized water) under hydrodynamic conditions (flow velocity, pressure,and the like) that purge solid salt from the flow field or otherstructure where it has formed. In some cases, deionized water is flowedthrough the cell under thermodynamic conditions that facilitatedissolution of the solid salt.

Management of Salt Concentration and Water Delivery in MEA Cells

As mentioned, salt may be provided to an MEA from various sourcesincluding anode water and preloaded MEA polymer layers. Salts providedto an MEA cell can become depleted over the course of the cell'soperation. This may happen even when salt-containing anode water isrecycled to the MEA. Various mechanisms may account for this loss. Forexample, salt from anode water may be taken up by one or more MEAcomponents such as a PEM or other cation exchange polymer layer.Further, some salt may move by diffusion, migration, and/or osmosis froma region of high concentration (anode) to a region of lowerconcentration (cathode). The anode water itself—not just its saltcontent—may move due to permeation of the anode water from the anode tothe cathode where it is swept away by flowing gaseous carbon oxide.

Various mechanisms may be employed to manage salt concentration duringoperation of an MEA cell. For example, anode water may be treated to (a)add salt, (b) remove impurities, and/or (c) add purified water. Suchtreatment may be accomplished by dosing concentrated salt solutionsand/or purified water to anode water in an anode water reservoir.Removing impurities may be accomplished by filtration and/or treatmentwith ion exchange resins.

Various mechanisms may be employed to manage anode water depletionduring operation of an MEA cell. One way is to capture the water thatleaves the anode and recirculate the water back to an anode water inlet.Another way is by recycling water recovered in the cathode productstream. In some implementations, the cathode water includes saltsintroduced via the anode water.

FIG. 1A provides an example of an electrolytic carbon oxide reductionsystem that may be used to control water composition and flow in an MEAcell. As shown in the figure, a system 101 includes an MEA cell 103comprising an anode 105, a cathode 107, and a membrane electrodeassembly 109. System 101 also includes an anode water recirculation loop111 and a gaseous carbon oxide recirculation loop 125.

In the depicted embodiment, anode water recirculation loop 111 deliverswater to and removes water from anode 105. Loop 111 includes an anodewater reservoir 113 and water flow paths 115, 117, and 119. Anode waterrecirculation loop 111 may interface with a water source 121 and/or asource of concentrated salt solution 123. These sources may be used todose the anode water with purified water and/or concentrated saltsolution in order to adjust the composition of the anode water. Incertain embodiments, water source 121 provides purified water such ashighly purified deionized water, e.g., water having a resistivity of atleast about 10 megaohm-cm. In the depicted embodiment, the source ofconcentrated salt solution 123 is directly connected to anode waterreservoir 113. In some embodiments, the source of concentrated saltsolution 123 is connected to another point on the anode waterrecirculation loop 111.

Not depicted in FIG. 1A is a water purification component such as afilter, a resin column, or other purifier configured to remove certainions such as iron ions or other transition metal ions from the anodewater. A water purification component may be provided in one of thewater flow paths 115, 117, or 119, or it may be provided with watersource 121, or even between water source 121 and anode waterrecirculation loop 111.

In certain embodiments, an anode water recirculation loop is configureddifferently from that shown in FIG. 1A. For example, an anode waterrecirculation loop may not include separate purified water andconcentrated salt sources. In some embodiments, a purified water sourceis not employed. In certain embodiments, both a purified water sourceand a concentrated salt solution source are directly connected to ananode water reservoir.

Regardless of which components are present in the anode waterrecirculation loop, the loop may be configured to provide or maintainanode water with a salt composition appropriate for operation of the MEAcell. Such salt compositions are described elsewhere herein.

Returning to FIG. 1A, gaseous carbon oxide recirculation loop 125provides a gaseous carbon oxide feed stream to and removes a gaseousproduct stream from the cathode 107. In addition to reaction products,cathode outlet stream may contain substantial quantities of unreactedgaseous carbon oxide. In the depicted embodiment, recirculation loop 125includes a water separator component 127 (e.g., a water condenser), areduction product recovery component 129, a humidifier 131, and, inaddition, flow paths 122, 124, 126, and 128. Fresh carbon oxide reactantgas may be provided from a carbon oxide source 133 which connects intogaseous carbon oxide recirculation loop 125.

Humidifier 131 may humidify an input stream of a carbon oxide gaseousreactant upstream from cathode 107. The humidifier provides carbon oxidewith a relatively high partial pressure of water vapor, which asexplained more fully herein may prevent drying the cathode 107 or anycomponents of MEA 109. In some embodiments, a humidifier is not presentin the system.

When the gaseous carbon oxide reactant contacts cathode 107, it mayremove anode water that has made its way from anode 105 through MEA 109,to cathode 107. In recirculation loop 125, anode water present in agaseous carbon oxide stream leaving the cathode 107 is brought incontact with water separator component 127 in which at least a fractionof the water present in the carbon oxide outlet stream is removed. Therelatively dried carbon oxide stream that leaves water separator 127enters the reduction product recovery component 129 which removes one ormore reduction products from the carbon oxide outlet stream. Suchreduction products may include carbon monoxide, hydrocarbons, and/orother organic compounds.

Some of the reduction product produced at the cathode of MEA cell 103may be dissolved or otherwise contained in water that is removed bywater separator 127. Optionally, to address this issue, the waterprocessed at separator 127 is provided to a reduction product recoverycomponent 135 configured to remove one or more reduction products fromthe water provided by water separator 127.

In some implementations, substantial quantities of anode water crossfrom the anode to the cathode of MEA 109 where the anode water, withsalts dissolved, can be lost. Given the high value of the salt and ofthe otherwise high purity water in which the salt is dissolved, aconnection between the two loops that can return water from gas loop 125to water loop 111 may improve the technical and commercial viability ofthe system. Note that, as explained, anode water may have extremely lowconcentrations (e.g., ppm or ppb levels) of certain inorganic and/ororganic materials. For example, the water may have extremely lowconcentrations of iron and other transition metal ions. The anode watermight also have intentionally added salts. Any such processed anodewater that can be recovered from the cathode side may be delivered backto the anode.

In the depicted embodiment, water that has been removed from the gaseouscarbon oxide recirculation loop 125 is delivered via a line 137 to theanode water reservoir 113 where it can reenter the anode waterrecirculation loop 111.

In alternative embodiments, there is not a direct connection betweencarbon oxide recirculation loop 125 and anode water recirculation loop111. Note also that reduction product recovery component 135 isoptional. In some implementations, reduction products in the waterrecirculation loop 125 are not removed and are included in the waterprovided to anode water recirculation loop 111. Some or all of thereduction products may be oxidized at the anode 105.

FIG. 1B illustrates an example of an electrolytic carbon oxide reductionsystem that may be used to control water composition and flow in an MEAcell. In the figure, a system 151 includes an MEA carbon oxide reductioncell 153 and two recirculation loops: a salt recirculation loop 155 anda pure water recirculation loop 157. Outputs of the two loops arecombined in a cell input reservoir 159, where they produce anode waterhaving a salt composition and concentration suitable for use with theMEA cell 153, e.g., a composition and concentration as describedelsewhere herein. Anode water is supplied from cell input reservoir 159to MEA cell 153 via a conduit 152.

Anode water that leaves MEA cell 153 is provided to a salt ion harvester161 configured to remove some or all the salt from the anode water.Relatively pure water leaves salt ion harvester 161 via a conduit 163that feeds the water to a water purifier component 165 that may removeremaining impurities after desirable salt ions have been harvested.Component 165 may include a small pore filter (e.g., a Milli-DI® filteravailable from Millipore Sigma of Darmstadt, Germany) and/or an ionexchange resin (e.g., in a bed). The resulting purified water may havevery low concentrations (e.g., ppm or ppb levels) of potentiallydetrimental ions such as transition metal ions and/or halides. Thepurified water that leaves water purifier 165 is provided to a reservoir169 via a conduit 167. The purified water in reservoir 169 is thenprovided, as needed, to cell input reservoir 159, where it is combinedwith salt or a concentrated salt solution to prepare anode water for usein the MEA cell 153. As illustrated, purified water is provided to cellinput reservoir 59 via a conduit 171.

As shown, the purified water recirculation loop 157 includes waterpurifier 165, reservoir 169, cell input reservoir 159, and salt ionharvester 161. In certain embodiments, a purified water loop does notinclude one or more of these elements. For example, some embodiments ofthe loop do not include a water purifier. Some embodiments of the loopdo not include a reservoir.

Returning to FIG. 1B, salt or concentrated salt solution produced bysalt ion harvester 161 is delivered via a conduit 175 to a saltreservoir 177, which maintains harvested salt in solid or solution form.Salt is provided, as needed, from reservoir 177 to cell input reservoir159, where it is combined with purified water to prepare anode water foruse in the MEA cell 153. Purified water is provided to cell inputreservoir 159 via a conduit 179. Salt reservoir 177 may serve as aholding point for desired salt ions to be pumped accordingly into cellinput reservoir 159.

As shown, the salt recirculation loop 155 includes reservoir 177 inaddition to cell input reservoir 159 and salt ion harvester 161. Incertain embodiments, a salt recirculation loop does not include one ormore of these elements. For example, some embodiments of the loop do notinclude a reservoir.

Examples of salt ion harvesters include devices that contain anion-selective membrane and devices configured with salt chelating andreleasing agents. Such devices may select for desired salt ions (e.g.,potassium or sodium ions) in an anode water stream. In certainembodiments, a salt ion harvester produces a solid salt precipitate thatis then selectively introduced back into a salt recirculation loop orother portion of an anode water management system.

As mentioned, water may be purified to remove detrimental ions by usingan ion exchange resin. Examples of such resins include (a) Diaion™CR11,available from Mitsubishi Chemical Corporate of Tokyo, Japan, whichcaptures relatively large multivalent ions (i.e., transition metals)that could deposit on the cathode catalyst and which releases sodiumions, and (b) Amberlite™ MB20, available from DuPont de Nemours, Inc. ofWilmington, Del., which captures all ions (cations and anions) andreleases only protons and hydroxide leaving very pure water. In certainembodiments, the resins are iminodiacetate chelating resins. In certainembodiments, the resins are mixtures of strong acid cation and strongbase anion exchange resins.

An electrolytic carbon oxide reduction system such as that depicted inFIGS. 1A and 1B may employ control system that includes a controller andone or more controllable components such as pumps, sensors, valves, andpower supplies. Examples of sensors include pressure sensors,temperature sensors, flow sensors, conductivity sensors, electrolytecomposition sensors including electrochemical instrumentation,chromatography systems, optical sensors such as absorbance measuringtools, and the like. Such sensors may be coupled to inlets and/oroutlets of an MEA cell (e.g., in a flow field), in a reservoir forholding anode water, purified water, salt solution, etc., and/or othercomponents of an electrolytic carbon oxide reduction system.

A control system may be configured to provide anode water over thecourse of an MEA cell's operation. For example, the control system maymaintain salt concentration at defined levels and/or recover andrecirculate anode water. Under control of the control system, the systemmay, for example, (a) recirculate anode water flowing out of an anode,(b) adjust the composition and/or flow rate of anode water into theanode, (c) move water from cathode outflow back to anode water, and/or(d) adjust the composition and/or flow rate of water recovered from thecathode stream, before returning to the anode. Note that the (d) mayaccount for carbon oxide reduction products in recovered water from thecathode. However, in some implementations, this need not be consideredas some reduction products may subsequently oxidize to harmless productsat the anode.

In certain embodiments, a control system is configured to utilizefeedback from sensors (e.g., conductivity/ion-specific monitoring) toadjust a mix of pure water and introduced salt ions to assure a bulkconductivity or other anode water parameter is within desired levels. Insome embodiments, sensors in the anode water detect the saltconcentration, and if the concentration becomes too low, salt from ahigher concentration reservoir may be added. If the salt concentrationbecomes too high, then pure water can be added to dilute the salt backto the desired concentration range.

A controller may include any number of processors and/or memory devices.The controller may contain control logic such software or firmwareand/or may execute instructions provided from another source. Acontroller may be integrated with electronics for controlling operationthe electrolytic cell before, during, and after reducing a carbon oxide.The controller may control various components or subparts of one ormultiple electrolytic carbon oxide reduction systems. The controller,depending on the processing requirements and/or the type of system, maybe programmed to control any of the processes disclosed herein, such asdelivery of gases, temperature settings (e.g., heating and/or cooling),pressure settings, power settings (e.g., electrical voltage and/orcurrent delivered to electrodes of an MEA cell), liquid flow ratesettings, fluid delivery settings, and dosing of purified water and/orsalt solution. These controlled processes may be connected to orinterfaced with one or more systems that work in concert with theelectrolytic carbon oxide reduction system.

In various embodiments, a controller comprises electronics havingvarious integrated circuits, logic, memory, and/or software that receiveinstructions, issue instructions, control operations described herein.The integrated circuits may include chips in the form of firmware thatstore program instructions, digital signal processors (DSPs), chipsdefined as application specific integrated circuits (ASICs), and/or oneor more microprocessors, or microcontrollers that execute programinstructions (e.g., software). Program instructions may be instructionscommunicated to the controller in the form of various individualsettings (or program files), defining operational parameters forcarrying out a process on one or more components of an electrolyticcarbon oxide reduction system. The operational parameters may, in someembodiments, be part of a recipe defined by process engineers toaccomplish one or more processing steps during generation of aparticular reduction product such as carbon monoxide, hydrocarbons,and/or other organic compounds.

The controller, in some implementations, may be a part of or coupled toa computer that is integrated with, coupled to the system, otherwisenetworked to the system, or a combination thereof. For example, thecontroller may utilize instructions stored remotely (e.g., in the“cloud”) and/or execute remotely. The computer may enable remote accessto the system to monitor current progress of electrolysis operations,examine a history of past electrolysis operations, examine trends orperformance metrics from a plurality of electrolysis operations, tochange parameters of current processing, to set processing steps tofollow a current processing, or to start a new process. In someexamples, a remote computer (e.g. a server) can provide process recipesto a system over a network, which may include a local network or theinternet. The remote computer may include a user interface that enablesentry or programming of parameters and/or settings, which are thencommunicated to the system from the remote computer. In some examples,the controller receives instructions in the form of data, which specifyparameters for each of the processing steps to be performed during oneor more operations.

The controller may be distributed, such as by comprising one or morediscrete controllers that are networked together and working towards acommon purpose, such as the processes and controls described herein. Anexample of a distributed controller for such purposes would be one ormore integrated circuits on an MEA cell or recirculation loop incommunication with one or more integrated circuits located remotely(such as at the platform level or as part of a remote computer) thatcombine to control a process on the chamber.

In certain embodiments, an electrolytic carbon oxide reduction system isconfigured and controlled to avoid precipitating salt within an MEA.Precipitated salt can block channels and/or have other impacts thatdegrade an MEA cell's performance. In some cases, a cell may become toodry, e.g., at the cathode side, because dry gaseous reactant removes toomuch water from the MEA, particularly on the cathode side. This issue,which may cause salt precipitation, may be addressed by controlling thewater partial pressure in the gas inlet stream (e.g., by humidifying thegaseous carbon oxide source gas). In some cases, a salt concentration inanode water is sufficiently high that it promotes salt precipitation inthe MEA. This issue may be addressed by controlling the saltconcentration in the anode water. In some embodiments, the system istaken offline periodically or as needed to address any actual orpotential salt build up in the MEA cell. While offline, the cathodecompartment or other portion of the system may be flushed with water toavoid or remove salt buildup.

MEA Design Embodiments MEA Overview

In various embodiments, an MEA contains an anode layer, a cathode layer,electrolyte, and optionally one or more other layers. The layers may besolids and/or gels. The layers may include polymers such asion-conducting polymers.

When in use, the cathode of an MEA promotes electrochemical reduction ofCO_(x) by combining three inputs: CO_(x), ions (e.g., protons) thatchemically react with CO_(x), and electrons. The reduction reaction mayproduce CO, hydrocarbons, and/or oxygen and hydrogen containing organiccompounds such as methanol, ethanol, and acetic acid. When in use, theanode of an MEA promotes an electrochemical oxidation reaction such aselectrolysis of water to produce elemental oxygen and protons. Thecathode and anode may each contain catalysts to facilitate theirrespective reactions.

The compositions and arrangements of layers in the MEA may promote highyield of a CO_(x) reduction products. To this end, the MEA mayfacilitate any one or more of the following conditions: (a) minimalparasitic reduction reactions (non-CO_(x) reduction reactions) at thecathode; (b) low loss of CO_(x) reactants at anode or elsewhere in theMEA; (c) maintain physical integrity of the MEA during the reaction(e.g., prevent delamination of the MEA layers); (d) prevent CO_(x)reduction product cross-over; (e) prevent oxidation production (e.g.,O₂) cross-over; (f) maintain a suitable environment at the cathode foroxidation; (g) provide pathway for desired ions to travel betweencathode and anode while blocking undesired ions; and (h) minimizevoltage losses. As explained herein, the presence of salts or salt ionsin the MEA can facilitate some of all of these conditions.

COx Reduction Considerations

Polymer-based membrane assemblies such as MEAs have been used in variouselectrolytic systems such as water electrolyzers and in various galvanicsystems such as fuel cells. However, CO_(x) reduction presents problemsnot encountered, or encountered to a lesser extent, in waterelectrolyzers and fuel cells.

For example, for many applications, an MEA for CO_(x) reduction requiresa lifetime on the order of about 50,000 hours or longer (approximatelyfive years of continuous operation), which is significantly longer thanthe expected lifespan of a fuel cell for automotive applications; e.g.,on the order of 5,000 hours. And for various applications, an MEA forCO_(x) reduction employs electrodes having a relatively large surfacearea by comparison to MEAs used for fuel cells in automotiveapplications. For example, MEAs for CO_(x) reduction may employelectrodes having surface areas (without considering pores and othernonplanar features) of at least about 500 cm².

CO_(x) reduction reactions may be implemented in operating environmentsthat facilitate mass transport of particular reactant and productspecies, as well as to suppress parasitic reactions. Fuel cell and waterelectrolyzer MEAs often cannot produce such operating environments. Forexample, such MEAs may promote undesirable parasitic reactions such asgaseous hydrogen evolution at the cathode and/or gaseous CO₂ productionat the anode.

In some systems, the rate of a CO_(x) reduction reaction is limited bythe availability of gaseous CO_(x) reactant at the cathode. By contrast,the rate of water electrolysis is not significantly limited by theavailability of reactant:liquid water tends to be easily accessible tothe cathode and anode, and electrolyzers can operate close to thehighest current density possible.

MEA Configurations

In certain embodiments, an MEA has a cathode layer, an anode layer, anda polymer electrolyte membrane (PEM) between the anode layer and thecathode layer. The polymer electrolyte membrane provides ioniccommunication between the anode layer and the cathode layer, whilepreventing electronic communication, which would produce a shortcircuit. The cathode layer includes a reduction catalyst and a firstion-conducting polymer. The cathode layer may also include an ionconductor and/or an electron conductor. The anode layer includes anoxidation catalyst and a second ion-conducting polymer. The anode layermay also include an ion conductor and/or an electron conductor. The PEMincludes a third ion-conducting polymer.

In certain embodiments, the MEA has a cathode buffer layer between thecathode layer and the polymer electrolyte membrane. The cathode bufferincludes a fourth ion-conducting polymer.

In certain embodiments, the MEA has an anode buffer layer between theanode layer and the polymer electrolyte membrane. The anode bufferincludes a fifth ion-conducting polymer.

In connection with certain MEA designs, there are three availableclasses of ion-conducting polymers: anion-conductors, cation-conductors,and mixed cation-and-anion-conductors. In certain embodiments, at leasttwo of the first, second, third, fourth, and fifth ion-conductingpolymers are from different classes of ion-conducting polymers.

Conductivity and Selectivity of Ion-Conducting Polymers for MEA Layers

The term “ion-conducting polymer” is used herein to describe a polymerelectrolyte having greater than about 1 mS/cm specific conductivity foranions and/or cations. The term “anion-conductor” describes anion-conducting polymer that conducts anions primarily (although therewill still be some small amount of cation conduction) and has atransference number for anions greater than about 0.85 at around 100micron thickness. The terms “cation-conductor” and/or “cation-conductingpolymer” describe an ion-conducting polymer that conducts cationsprimarily (e.g., there can still be an incidental amount of anionconduction) and has a transference number for cations greater thanapproximately 0.85 at around 100 micron thickness. For an ion-conductingpolymer that is described as conducting both anions and cations (a“cation-and-anion-conductor”), neither the anions nor the cations has atransference number greater than approximately 0.85 or less thanapproximately 0.15 at around 100 micron thickness. To say a materialconducts ions (anions and/or cations) is to say that the material is anion-conducting material or ionomer. Examples of ion-conducting polymersof each class are provided in the below Table.

Ion-Conducting Polymers Common Class Description Features Examples A.Anion- Greater than Positively aminated conducting approximately chargedtetramethyl 1 mS/cm specific functional polyphenylene; conductivity forgroups are poly(ethylene-co- anions, which covalently tetrafluoro- havea bound to the ethylene)- transference polymer based quaternary numbergreater backbone ammonium polymer; than quaternized approximatelypolysulfone 0.85 at around 100 micron thickness B. Conducts Greater thanSalt is polyethylene oxide; both anions approximately soluble inpolyethylene and cations 1 mS/cm the polymer glycol; conductivity andthe salt poly(vinylidene for ions ions can fluoride); (including movethrough polyurethane both cations and the polymer anions), whichmaterial have a transference number between approximately 0.15 and 0.85at around 100 micron thickness C. Cation- Greater than Negativelyperfluorosulfonic conducting approximately charged acid 1 mS/cmfunctional polytetra- specific groups are fluoroethylene conductivitycovalently co-polymer; for cations, bound to sulfonated which have a thepolymer poly(ether transference backbone ether ketone); number greaterpoly(styrene than sulfonic acid- approximately co-maleic acid) 0.85 ataround 100 micron thickness

Some Class A ion-conducting polymers are known by tradenames such as2259-60 (Pall RAI), AHA by Tokuyama Co, Fumasep® FAA-(fumatech GbbH),Sustanion®, Morgane ADP by Solvay, or Tosflex® SF-17 by Tosoh anionexchange membrane material. Further class A ion-conducting polymersinclude HNN5/HNN8 by Ionomr, FumaSep by Fumatech, TM1 by Orion, andPAP-TP by W7energy. Some Class C ion-conducting polymers are known bytradenames such as various formulations of Nafion® (DuPont™),GORE-SELECT® (Gore), Fumapem® (fumatech GmbH), and Aquivion® PFSA(Solvay).

Bipolar MEA for COx Reduction

In certain embodiments, the MEA includes a bipolar interface with ananion-conducting polymer on the cathode side of the MEA and aninterfacing cation-conducting polymer on the anode side of the MEA. Insome implementations, the cathode contains a first catalyst and ananion-conducting polymer. In certain embodiments, the anode contains asecond catalyst and a cation-conducting polymer. In someimplementations, a cathode buffer layer, located between the cathode andPEM, contains an anion-conducting polymer. In some embodiments, an anodebuffer layer, located between the anode and PEM, contains acation-conducting polymer.

During operation, an MEA with a bipolar interface moves ions through apolymer-electrolyte, moves electrons through metal and/or carbon in thecathode and anode layers, and moves liquids and gas through pores in thelayers.

In embodiments employing an anion-conducting polymer in the cathodeand/or in a cathode buffer layer, the MEA can decrease or block unwantedreactions that produce undesired products and decrease the overallefficiency of the cell. In embodiments employing a cation-conductingpolymer in the anode and/or in an anode buffer layer can decrease orblock unwanted reactions that reduce desired product production andreduce the overall efficiency of the cell.

For example, at levels of electrical potential used for cathodicreduction of CO₂, hydrogen ions may be reduced to hydrogen gas. This isa parasitic reaction; current that could be used to reduce CO₂ is usedinstead to reduce hydrogen ions. Hydrogen ions may be produced byvarious oxidation reactions performed at the anode in a CO₂ reductionreactor and may move across the MEA and reach the cathode where they canbe reduced to produce hydrogen gas. The extent to which this parasiticreaction can proceed is a function of the concentration of hydrogen ionspresent at the cathode. Therefore, an MEA may employ an anion-conductingmaterial in the cathode layer and/or in a cathode buffer layer. Theanion-conducting material at least partially blocks hydrogen ions fromreaching catalytic sites on the cathode. As a result, parasiticproduction of hydrogen gas generation is decreased and the rate of CO orother product production and the overall efficiency of the process areincreased.

Another reaction that may be avoided is reaction of carbonate orbicarbonate ions at the anode to produce CO₂. Aqueous carbonate orbicarbonate ions may be produced from CO₂ at the cathode. If such ionsreach the anode, they may react with hydrogen ions to produce andrelease gaseous CO₂. The result is net movement of CO₂ from the cathodeto the anode, where it does not react and is lost with oxidationproducts. To prevent the carbonate and bicarbonate ion produced at thecathode from reaching the anode, the anode and/or a anode buffer layermay include a cation-conducting polymer, which at least partially blocksthe transport of negative ions such as bicarbonate ions to the anode.

Thus, in some designs, a bipolar membrane structure raises the pH at thecathode to facilitate CO₂ reduction while a cation-conducting polymersuch as a proton-exchange layer prevents the passage of significantamounts of CO₂ and CO₂ reduction products (e.g., bicarbonate) to theanode side of the cell.

An example MEA 200 for use in CO_(x) reduction is shown in FIG. 2. TheMEA 200 has a cathode layer 220 and an anode layer 240 separated by anion-conducting polymer layer 260 that provides a path for ions to travelbetween the cathode layer 220 and the anode layer 240. In certainembodiments, the cathode layer 220 includes an anion-conducting polymerand/or the anode layer 240 includes a cation-conducting polymer. Incertain embodiments, the cathode layer and/or the anode layer of the MEAare porous. The pores may facilitate gas and/or fluid transport and mayincrease the amount of catalyst surface area that is available forreaction.

The ion-conducting layer 260 may include two or three sublayers: apolymer electrolyte membrane (PEM) 265, an optional cathode buffer layer225, and/or an optional anode buffer layer 245. One or more layers inthe ion-conducting layer may be porous. In certain embodiments, at leastone layer is nonporous so that reactants and products of the cathodecannot pass via gas and/or liquid transport to the anode and vice versa.In certain embodiments, the PEM layer 265 is nonporous. Examplecharacteristics of anode buffer layers and cathode buffer layers areprovided elsewhere herein. In certain embodiments, the ion-conductinglayer includes only a single layer or two sublayers.

FIG. 3 shows CO₂ electrolyzer 303 configured to receive water and CO₂(e.g., humidified or dry gaseous CO₂) as a reactant at a cathode 305 andexpel CO as a product. Electrolyzer 303 is also configured to receivewater as a reactant at an anode 307 and expel gaseous oxygen.Electrolyzer 303 includes bipolar layers having an anion-conductingpolymer 309 adjacent to cathode 305 and a cation-conducting polymer 311(illustrated as a proton-exchange membrane) adjacent to anode 307.

As illustrated in the magnification inset of a bipolar interface 313 inelectrolyzer 303, the cathode 305 includes an anion exchange polymer(which in this example is the same anion-conducting polymer 309 that isin the bipolar layers) electronically conducting carbon supportparticles 317, and metal nanoparticles 319 supported on the supportparticles. CO₂ and water are transported via pores such as pore 321 andreach metal nanoparticles 319 where they react, in this case withhydroxide ions, to produce bicarbonate ions and reduction reactionproducts (not shown). CO₂ may also reach metal nanoparticles 319 bytransport within anion exchange polymer 315.

Hydrogen ions are transported from anode 307, and through thecation-conducting polymer 311, until they reach bipolar interface 313,where they are hindered from further transport toward the cathode byanion exchange polymer 309. At interface 313, the hydrogen ions mayreact with bicarbonate or carbonate ions to produce carbonic acid(H₂CO₃), which may decompose to produce CO₂ and water. As explainedherein, the resulting CO₂ may be provided in gas phase and should beprovided with a route in the MEA back to the cathode 305 where it can bereduced. The cation-conducting polymer 311 hinders transport of anionssuch as bicarbonate ions to the anode where they could react withprotons and release CO₂, which would be unavailable to participate in areduction reaction at the cathode.

As illustrated, a cathode buffer layer having an anion-conductingpolymer may work in concert with the cathode and its anion-conductivepolymer to block transport of protons to the cathode. While MEAsemploying ion conducting polymers of appropriate conductivity types inthe cathode, the anode, cathode buffer layer, and if present, an anodebuffer layer may hinder transport of cations to the cathode and anionsto the anode, cations and anions may still come in contact in the MEA'sinterior regions, such as in the membrane layer.

As illustrated in FIG. 3, bicarbonate and/or carbonate ions combine withhydrogen ions between the cathode layer and the anode layer to formcarbonic acid, which may decompose to form gaseous CO₂. It has beenobserved that MEAs sometime delaminate, possibly due to this productionof gaseous CO₂, which does not have an easy egress path.

The delamination problem can be addressed by employing a cathode bufferlayer having inert filler and associated pores. One possible explanationof its effectiveness is that the pores create paths for the gaseouscarbon dioxide to escape back to the cathode where it can be reduced. Insome embodiments, the cathode buffer layer is porous but at least onelayer between the cathode layer and the anode layer is nonporous. Thiscan prevent the passage of gases and/or bulk liquid between the cathodeand anode layers while still preventing delamination. For example, thenonporous layer can prevent the direct passage of water from the anodeto the cathode. The porosity of various layers in an MEA is describedfurther at other locations herein.

Examples of Bipolar MEAs

As an example, an MEA includes a cathode layer including a reductioncatalyst and a first anion-conducting polymer (e.g., Sustainion, FumaSepFAA-3, Tokuyama anion exchange polymer), an anode layer including anoxidation catalyst and a first cation-conducting polymer (e.g., PFSApolymer), a membrane layer including a second cation-conducting polymerand arranged between the cathode layer and the anode layer toconductively connect the cathode layer and the anode layer, and acathode buffer layer including a second anion-conducting polymer (e.g.,Sustainion, FumaSep FAA-3, Tokuyama anion exchange polymer) and arrangedbetween the cathode layer and the membrane layer to conductively connectthe cathode layer and the membrane layer. In this example, the cathodebuffer layer can have a porosity between about 1 and 90 percent byvolume, but can additionally or alternatively have any suitable porosity(including, e.g., no porosity). In other examples the cathode bufferlayer can have any suitable porosity (e.g., between 0.01-95%, 0.1-95%,0.01-75%, 1-95%, 1-90%, etc.).

Too much porosity can lower the ionic conductivity of the buffer layer.In some embodiments, the porosity is 20% or below, and in particularembodiments, between 0.1-20%, 1-10%, or 5-10%. Porosity in these rangescan be sufficient to allow movement of water and/or CO₂ without losingionic conductivity. Porosity may be measured as described further below.

In a related example, the membrane electrode assembly can include ananode buffer layer that includes a third cation-conducting polymer, andis arranged between the membrane layer and the anode layer toconductively connect the membrane layer and the anode layer. The anodebuffer layer preferably has a porosity between about 1 and 90 percent byvolume, but can additionally or alternatively have any suitable porosity(including, e.g., no porosity). However, in other arrangements andexamples, the anode buffer layer can have any suitable porosity (e.g.,between 0.01-95%, 0.1-95%, 0.01-75%, 1-95%, 1-90%). As with the cathodebuffer layer, in some embodiments, the porosity is 20% or below, e.g.0.1-20%, 1-10%, or 5-10%

In an example, an anode buffer layer may be used in a MEA having acathode catalyst layer with anion exchange polymer, a cathode bufferlayer with anion-exchange polymer, a membrane with cation-exchangepolymer, and an anode buffer layer with anion-exchange polymer. In sucha structure, the anode buffer layer may porous to facilitate watertransport to the membrane/anode buffer layer interface. Water will besplit at this interface to make protons that travel through the membraneand hydroxide that travels to the anode catalyst layer. One advantage ofthis structure is the potential use of low cost water oxidationcatalysts (e.g., NiFeO_(x)) that are only stable in basic conditions.

In another specific example, the membrane electrode assembly includes acathode layer including a reduction catalyst and a firstanion-conducting polymer (e.g., Sustainion, FumaSep FAA-3, Tokuyamaanion exchange polymer), an anode layer including an oxidation catalystand a first cation-conducting polymer, a membrane layer including asecond anion-conducting polymer (e.g., Sustainion, FumaSep FAA-3,Tokuyama anion exchange polymer) and arranged between the cathode layerand the anode layer to conductively connect the cathode layer and theanode layer, and an anode buffer layer including a secondcation-conducting polymer and arranged between the anode layer and themembrane layer to conductively connect the anode layer and the membranelayer.

An MEA containing an anion-exchange polymer membrane and an anode bufferlayer containing cation-exchange polymer may be used for CO reduction.In this case, water would form at the membrane/anode buffer layerinterface. Pores in the anode buffer layer could facilitate waterremoval. One advantage of this structure would be the use of an acidstable (e.g., IrO_(x)) water oxidation catalyst.

In a related example, the membrane electrode assembly can include acathode buffer layer that includes a third anion-conducting polymer, andis arranged between the cathode layer and the membrane layer toconductively connect the cathode layer and the membrane layer. The thirdanion-conducting polymer can be the same or different from the firstand/or second anion-conducting polymer. The cathode buffer layerpreferably has a porosity between about 1 and 90 percent by volume, butcan additionally or alternatively have any suitable porosity (including,e.g., no porosity). However, in other arrangements and examples, thecathode buffer layer can have any suitable porosity (e.g., between0.01-95%, 0.1-95%, 0.01-75%, 1-95%, 1-90%). In some embodiments, theporosity is 20% or below, and in particular embodiments, between0.1-20%, 1-10%, or 5-10%.

In an example, a cathode catalyst layer composed of Au nanoparticles 4nm in diameter supported on Vulcan XC72R carbon and mixed with TM1(mTPN-1) anion exchange polymer electrolyte (from Orion). Layer is ˜15um thick, Au/(Au+C)=20 wt %, TM1 to catalyst mass ratio of 0.32, massloading of 1.4-1.6 mg/cm2 (total Au+C), estimated porosity of 0.56.Anion-exchange polymer layer composed of TM1 and PTFE particles. PTFE isapproximately 200 nm in diameter. TM1 molecular weight is 30 k-45 k.Thickness of the layer is ˜15 um. PTFE may introduce porosity of about8%. Proton-exchange membrane layer composed of perfluorosulfonic acidpolymer (e.g., Nafion 117). Thickness is approximately 125 um. Membraneforms a continuous layer that prevents significant movement of gas (CO₂,CO, H₂) through the layer. Anode catalyst layer composed of Ir or IrOxnanoparticles (100-200 nm aggregates) that is 10 um thick.

Anion Exchange Membrane-Only MEA for CO_(x) Reduction

In some embodiments, an MEA does not contain a cation-conducting polymerlayer. In such embodiments, the electrolyte is not a cation-conductingpolymer and the anode, if it includes an ion-conducting polymer, doesnot contain a cation-conducting polymer. Examples are provided herein.

An AEM-only MEA allows conduction of anions across the MEA. Inembodiments in which none of the MEA layers has significant conductivityfor cations, hydrogen ions have limited mobility in the MEA. In someimplementations, an AEM-only membrane provides a high pH environment(e.g., at least about pH 7) and may facilitate CO₂ and/or CO reductionby suppressing the hydrogen evolution parasitic reaction at the cathode.As with other MEA designs, the AEM-only MEA allows ions, notably anionssuch as hydroxide ions, to move through polymer-electrolyte. The pH maybe lower in some embodiments; a pH of 4 or greater may be high enough tosuppress hydrogen evolution. The AEM-only MEA also permits electrons tomove to and through metal and carbon in catalyst layers. In embodiments,having pores in the anode layer, the cathode layer, and/or the PEM, theAEM-only MEA permits liquids and gas to move through pores.

In certain embodiments, the AEM-only MEA comprises an anion-exchangepolymer electrolyte membrane with an electrocatalyst layer on eitherside: a cathode and an anode. In some embodiments, one or bothelectrocatalyst layers also contain anion-exchange polymer-electrolyte.

In certain embodiments, an AEM-only MEA is formed by depositing cathodeand anode electrocatalyst layers onto porous conductive supports such asgas diffusion layers to form gas diffusion electrodes (GDEs), andsandwiching an anion-exchange membrane between the gas diffusionelectrodes.

In certain embodiments, an AEM-only MEA is used for CO₂ reduction. Theuse of an anion-exchange polymer electrolyte avoids low pH environmentthat disfavors CO₂ reduction. Further, water is transported away fromthe cathode catalyst layer when an AEM is used, thereby preventing waterbuild up (flooding) which can block reactant gas transport in thecathode of the cell.

Water transport in the MEA occurs through a variety of mechanisms,including diffusion and electro-osmotic drag. In some embodiments, atcurrent densities of the CO₂ electrolyzers described herein,electro-osmotic drag is the dominant mechanism. Water is dragged alongwith ions as they move through the polymer electrolyte. For acation-exchange membrane such as Nafion membrane, the amount of watertransport is well characterized and understood to rely on thepre-treatment/hydration of the membrane. Protons move from positive tonegative potential (anode to cathode) with. each carrying 2-4 watermolecules with it, depending on pretreatment. In anion-exchangepolymers, the same type of effect occurs. Hydroxide, bicarbonate, orcarbonate ions moving through the polymer electrolyte will ‘drag’ watermolecules with them. In the anion-exchange MEAs, the ions travel fromnegative to positive voltage, so from cathode to anode, and they carrywater molecules with them, moving water from the cathode to the anode inthe process.

In certain embodiments, an AEM-only MEA is employed in CO reductionreactions. Unlike the CO₂ reduction reaction, CO reduction does notproduce carbonate or bicarbonate anions that could transport to theanode and release valuable reactant.

FIG. 4 illustrates an example construction of a CO₂ reduction MEA 401having a cathode catalyst layer 403, an anode catalyst layer 405, and ananion-conducting PEM 407. In certain embodiments, cathode catalyst layer403 includes metal catalyst particles (e.g., nanoparticles) that areunsupported or supported on a conductive substrate such as carbonparticles. In some implementations, cathode catalyst layer 403additionally includes an anion-conducting polymer. The metal catalystparticles may catalyze CO₂ reduction, particularly at pH greater than 7.In certain embodiments, anode catalyst layer 405 includes metal oxidecatalyst particles (e.g., nanoparticles) that are unsupported orsupported on a conductive substrate such as carbon particles. In someimplementations, anode catalyst layer 403 additionally includes ananion-conducting polymer. Examples of metal oxide catalyst particles foranode catalyst layer 405 include iridium oxide, nickel oxide, nickeliron oxide, iridium ruthenium oxide, platinum oxide, and the like.Anion-conducting PEM 407 may comprise any of various anion-conductingpolymers such as, for example, HNN5/HNN8 by Ionomr, FumaSep by Fumatech,TM1 by Orion, PAP-TP by W7energy, Sustainion by Dioxide Materials, andthe like. These and other anion-conducting polymer that have an ionexchange capacity (IEC) ranging from 1.1 to 2.6, working pH ranges from0-14, bearable solubility in some organic solvents, reasonable thermalstability and mechanical stability, good ionic conductivity/ASR andacceptable water uptake/swelling ratio may be used. The polymers may bechemically exchanged to certain anions instead of halogen anions priorto use.

As illustrated in FIG. 4, CO₂ such as CO₂ gas may be provided to cathodecatalyst layer 403. In certain embodiments, the CO₂ may be provided viaa gas diffusion electrode. At the cathode catalyst layer 403, the CO₂reacts to produce reduction product indicated generically asC_(x)O_(y)H_(z). Anions produced at the cathode catalyst layer 403 mayinclude hydroxide, carbonate, and/or bicarbonate. These may diffuse,migrate, or otherwise move to the anode catalyst layer 405. At the anodecatalyst layer 405, an oxidation reaction may occur such as oxidation ofwater to produce diatomic oxygen and hydrogen ions. In someapplications, the hydrogen ions may react with hydroxide, carbonate,and/or bicarbonate to produce water, carbonic acid, and/or CO₂. Fewerinterfaces give lower resistance. In some embodiments, a highly basicenvironment is maintained for C2 and C3 hydrocarbon synthesis.

FIG. 5 illustrates an example construction of a CO reduction MEA 501having a cathode catalyst layer 503, an anode catalyst layer 505, and ananion-conducting PEM 507. Overall, the constructions of MEA 501 may besimilar to that of MEA 401 in FIG. 4. However, the cathode catalyst maybe chosen to promote a CO reduction reaction, which means that differentreduction catalysts would be used in CO and CO₂ reduction embodiments.

In some embodiments, an AEM-only MEA may be advantageous for COreduction. The water uptake number of the AEM material can be selectedto help regulate moisture at the catalyst interface, thereby improvingCO availability to the catalyst. AEM-only membranes can be favorable forCO reduction due to this reason. Bipolar membranes can be more favorablefor CO₂ reduction due to better resistance to CO₂ dissolving andcrossover in basic anolyte media.

In various embodiments, cathode catalyst layer 503 includes metalcatalyst particles (e.g., nanoparticles) that are unsupported orsupported on a conductive substrate such as carbon particles. In someimplementations, cathode catalyst layer 503 additionally includes ananion-conducting polymer. In certain embodiments, anode catalyst layer505 includes metal oxide catalyst particles (e.g., nanoparticles) thatare unsupported or supported on a conductive substrate such as carbonparticles. In some implementations, anode catalyst layer 503additionally includes an anion-conducting polymer. Examples of metaloxide catalyst particles for anode catalyst layer 505 may include thoseidentified for the anode catalyst layer 405 of FIG. 4. Anion-conductingPEM 507 may comprise any of various anion-conducting polymer such as,for example, those identified for the PEM 407 of FIG. 4.

As illustrated in FIG. 5, CO gas may be provided to cathode catalystlayer 503. In certain embodiments, the CO may be provided via a gasdiffusion electrode. At the cathode catalyst layer 503, the CO reacts toproduce reduction product indicated generically as C_(x)O_(y)H_(z).

Anions produced at the cathode catalyst layer 503 may include hydroxideions. These may diffuse, migrate, or otherwise move to the anodecatalyst layer 505. At the anode catalyst layer 505, an oxidationreaction may occur such as oxidation of water to produce diatomic oxygenand hydrogen ions. In some applications, the hydrogen ions may reactwith hydroxide ions to produce water.

While the general configuration of the MEA 501 is similar to that of MEA401, there are certain differences in the MEAs. First, MEAs may bewetter for CO reduction, helping the catalyst surface to have more —H.Also, for CO₂ reduction, a significant amount of CO₂ may be dissolvedand then transferred to the anode for an AEM-only MEA such as shown inFIG. 4. For CO reduction, there is less likely to be significant CO gascrossover. In this case, the reaction environment could be very basic.MEA materials, including the catalyst, may be selected to have goodstability in high pH environment. In some embodiments, a thinnermembrane may be used for CO reduction than for CO₂ reduction.

Example of AM-Only MEA

1. Copper metal (USRN 40 nm thick Cu, ˜0.05 mg/cm²) was deposited onto aporous carbon sheet (Sigracet 39BC gas diffusion layer) via electronbeam deposition. Ir metal nanoparticles were deposited onto a poroustitanium sheet at a loading of 3 mg/cm² via drop casting. Ananion-exchange membrane from Ionomr (25-50 μm, 80 mS/cm² OH—conductivity, 2-3 mS/cm² HCO₃ ⁻ conductivity, 33-37% water uptake) wassandwiched between the porous carbon and titanium sheets with theelectrocatalyst layers facing the membrane.

2. Sigma Aldrich 80 nm spherical Cu nanoparticles, mixed with FAA-3anion exchange solid polymer electrolyte from Fumatech, FAA-3 tocatalyst mass ratio of 0.10, setup as described above.

US Patent Application Publication No. US 2017/0321334, published Nov. 9,2017 and US Patent Application Publication No. 20190226103, publishedJul. 25, 2019, which describe various features and examples of MEAS, areincorporated herein by reference in their entireties. All publicationsreferred to herein are incorporated by reference in their entireties asif fully set forth herein.

Cathode Catalyst Layer—General Structure

As indicated above, the cathode of the MEA, which is also referred to asthe cathode layer or cathode catalyst layer, facilitates CO_(x)conversion. It is a porous layer containing catalysts for CO_(x)reduction reactions.

In some embodiments, the cathode catalyst layer contains a blend ofreduction catalyst particles, electronically-conductive supportparticles that provide support for the reduction catalyst particles, anda cathode ion-conducting polymer. In some embodiments, the reductioncatalyst particles are blended with the cathode ion-conducting polymerwithout a support.

Examples of materials that can be used for the reduction catalystparticles include, but are not limited, to transition metals such as V,Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Au, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W,Re, Ir, Pt, and Hg, and combinations thereof, and/or any other suitablematerials. Other catalyst materials can include alkali metals, alkalineearth metals, lanthanides, actinides, and post transition metals, suchas Sn, Si, Ga, Pb, Al, Tl, Sb, Te, Bi, Sm, Tb, Ce, Nd and In orcombinations thereof, and/or any other suitable catalyst materials. Thechoice of catalyst depends on the particular reaction performed at thecathode of the CRR.

Catalysts can be in the form of nanoparticles that range in size fromapproximately 1 to 100 nm or particles that range in size fromapproximately 0.2 to 10 nm or particles in the size range ofapproximately 1-1000 nm or any other suitable range. In addition tonanoparticles and larger particles, films and nanostructured surfacesmay be used.

If used, the electronically-conductive support particles in the cathodecan be carbon particles in various forms. Other possible conductivesupport particles include boron-doped diamond or fluorine-doped tinoxide. In one arrangement, the conductive support particles are Vulcancarbon. The conductive support particles can be nanoparticles. The sizerange of the conductive support particles is between approximately 20 nmand 1000 nm or any other suitable range. It is especially useful if theconductive support particles are compatible with the chemicals that arepresent in the cathode when the CRR is operating, are reductivelystable, and have a high hydrogen production overpotential so that theydo not participate in any electrochemical reactions.

For composite catalysts such as Au/C, example metal nanoparticle sizesmay range from about 2 nm-20 nm and the carbon size may be from about20-200 nm as supporting materials. For pure metal catalyst such as Ag orCu, the particles have a board range from 2 nm to 500 nm in term ofcrystal grain size. The agglomeration could be even larger to micrometerrange.

In general, such conductive support particles are larger than thereduction catalyst particles, and each conductive support particle cansupport many reduction catalyst particles. FIG. 6 is a schematic drawingthat shows a possible morphology for two different kinds of catalystssupported on a catalyst support particle 610, such as a carbon particle.Catalyst particles 630 of a first type and second catalyst particles 650of a second type are attached to the catalyst support particle 610. Invarious arrangements, there is only one type of catalyst particle orthere are more than two types of catalyst particles attached to thecatalyst support particle 610.

Using two types of catalysts may be useful in certain embodiments. Forexample, one catalyst may be good at one reaction (e.g., CO₂→CO) and thesecond good at another reaction (e.g., CO→CH₄). Overall, the catalystlayer would perform the transformation of CO₂ to CH₄, but differentsteps in the reaction would take place on different catalysts.

The electronically-conductive support may also be in forms other thanparticles, including tubes (e.g., carbon nanotubes) and sheets (e.g.,graphene). Structures having high surface area to volume are useful toprovide sites for catalyst particles to attach.

In addition to reduction catalyst particles andelectronically-conductive support particles, the cathode catalyst layermay include an ion conducting polymer. There are tradeoffs in choosingthe amount of cathode ion-conducting polymer in the cathode. It can beimportant to include enough cathode ion-conducting polymer to providesufficient ionic conductivity. But it is also important for the cathodeto be porous so that reactants and products can move through it easilyand to maximize the amount of catalyst surface area that is availablefor reaction. In various arrangements, the cathode ion-conductingpolymer makes up somewhere in the range between 30 and 70 wt %, between20 and 80 wt %, or between 10 and 90 wt %, of the material in thecathode layer, or any other suitable range. The wt % of ion-conductingpolymer in the cathode is selected to result in the cathode layerporosity and ion-conductivity that gives the highest current density forCO_(x) reduction. In some embodiments, it may be between 20 and 60 wt. %or between 20 and 50 wt. %. Example thicknesses of the cathode catalystlayer range from about 80 nm-300 μm.

In addition to the reduction catalyst particles, cathode ion conductingpolymer, and if present, the electronically-conductive support, thecathode catalyst layer may include other additives such as PTFE.

In addition to polymer:catalyst mass ratios, the catalyst layer may becharacterized by mass loading (mg/cm²), and porosity. Porosity may bedetermined by a various manners. In one method, the loading of eachcomponent (e.g., catalyst, support, and polymer) is multiplied by itsrespective density. These are added together to determine the thicknessthe components take up in the material. This is then divided by thetotal known thickness to obtain the percentage of the layer that isfilled in by the material. The resulting percentage is then subtractedfrom 1 to obtain the percentage of the layer assumed to be filled withair, which is the porosity. Methods such as mercury porosimetry or imageprocessing on TEM images may be used as well.

Examples of cathode catalyst layers for CO, methane, andethylene/ethanol productions are given below.

-   -   CO production: Au nanoparticles 4 nm in diameter supported on        Vulcan XC72R carbon and mixed with TM1 anion exchange polymer        electrolyte from Orion. Layer is about 15 μm thick,        Au/(Au+C)=30%, TM1 to catalyst mass ratio of 0.32, mass loading        of 1.4-1.6 mg/cm², estimated porosity of 0.47    -   Methane production: Cu nanoparticles of 20-30 nm size supported        on Vulcan XC72R carbon, mixed with FAA-3 anion exchange solid        polymer electrolyte from Fumatech. FAA-3 to catalyst mass ratio        of 0.18. Estimated Cu nanoparticle loading of ˜7.1 μg/cm²,        within a wider range of 1-100 μg/cm²    -   Ethylene/ethanol production: Cu nanoparticles of 25-80 nm size,        mixed with FAA-3 anion exchange solid polymer electrolyte from        Fumatech. FAA-3 to catalyst mass ratio of 0.10. Deposited either        on Sigracet 39BC GDE for pure AEM or on MEA electrode assembly.        Estimated Cu nanoparticle loading of 270 μg/cm².

The functions, materials, and structures of the components of thecathode catalyst layer are described further below.

Water Management (Cathode Catalyst Layer)

The cathode catalyst layer may facilitate movement of water to preventit from being trapped in the cathode catalyst layer. Trapped water canhinder access of CO_(x) to the catalyst and/or hinder movement ofreaction product out of the cathode catalyst layer.

Water management challenges are in many respects unique to CRRs. Forexample, compared to a PEM fuel cell's oxygen electrode, a CRR uses amuch lower gas flow rate. Vapor phase water removal is determined by thevolumetric gas flow, thus much less vapor phase water removal is carriedout in a CRR. A CRR may also operate at higher pressure (e.g., 100psi-450 psi) than a fuel cell; at higher pressure the same molar flowresults in lower volumetric flow and lower vapor phase water removal. Asa result, liquid water in MEA of a CRR is present to be removed. Forsome MEAs, the ability to remove vapor phase water is further limited bytemperature limits not present in fuel cells. For example, CO₂ to COreduction may be performed at about 50° C. and ethylene and methaneproduction may be performed at 20° C.-25° C. This is compared to typicaloperating temperatures of 80° C. to 120° C. for fuel cells. As a result,there is more liquid phase water to remove.

Properties that affect ability of the cathode catalyst layer to removewater include porosity; pore size; distribution of pore sizes;hydrophobicity; the relative amounts of ion conducting polymer, metalcatalyst particles, and electronically-conductive support; the thicknessof the layer; the distribution of the catalyst throughout the layer; andthe distribution of the ion conducting polymer through the layer andaround the catalyst.

A porous layer allows an egress path for water. In some embodiments, thecathode catalyst layer has a pore size distribution that includes poreshaving sizes of 1 nm-100 nm and pores having sizes of at least 1 micron.This size distribution can aid in water removal. The porous structurescould be formed by one or more of: pores within the carbon supportingmaterials; stacking pores between stacked spherical carbonnanoparticles; secondary stacking pores between agglomerated carbonspheres (micrometer scale); or inert filler (e.g., PTFE) introducedporous with the interface between the PTFE and carbon also creatingirregular pores ranging from hundreds of nm to micrometers.

The cathode catalyst layer may have a thickness that contributes towater management. Using a thicker layer allows the catalyst and thus thereaction to be distributed in a larger volume. This spreads out thewater distribution and makes it easier to manage.

Ion-conducting polymers having non-polar, hydrophobic backbones may beused in the cathode catalyst layer. In some embodiments, the cathodecatalyst layer may include a hydrophobic polymer such as PTFE inaddition to the ion-conducting polymer. In some embodiments, theion-conducting polymer may be a component of a co-polymer that alsoincludes a hydrophobic polymer.

Gas Transport (Cathode Catalyst Layer)

The cathode catalyst layer may be structured for gas transport.Specifically, CO_(x) is transported to the catalyst and gas phasereaction products (e.g., CO, ethylene, methane, etc.) is transported outof the catalyst layer.

Certain challenges associated with gas transport are unique to CRRs. Gasis transported both in and out of the cathode catalyst layer—CO_(x) inand products such as CO, ethylene, and methane out. In a PEM fuel cell,gas (O₂ or H₂) is transported in but nothing or product water comes out.And in a PEM water electrolyzer, water is the reactant with O₂ and H₂gas products.

Operating conditions including pressures, temperature, and flow ratethrough the reactor affect the gas transport. Properties of the cathodecatalyst layer that affect gas transport include porosity; pore size anddistribution; layer thickness; and ionomer distribution.

In some embodiments, the ionomer-catalyst contact is minimized. Forexample, in embodiments that use a carbon support, the ionomer may forma continuous network along the surface of the carbon with minimalcontact with the catalyst. The ionomer, support, and catalyst may bedesigned such that the ionomer has a higher affinity for the supportsurface than the catalyst surface. This can facilitate gas transport toand from the catalyst without being blocked by the ionomer, whileallowing the ionomer to conduct ions to and from the catalyst.

Ionomer (Cathode Catalyst Layer)

The ionomer may have several functions including holding particles ofthe catalyst layer together and allowing movement of ions through thecathode catalyst layer. In some cases, the interaction of the ionomerand the catalyst surface may create an environment favorable for CO_(x)reduction, increasing selectivity to a desired product and/or decreasingthe voltage required for the reaction. Importantly, the ionomer is anion-conducting polymer to allow for the movement of ions through thecathode catalyst layer. Hydroxide, bicarbonate, and carbonate ions, forexample, are moved away from the catalyst surface where the CO_(x)reduction occurs. In the description below, the ionomer in the cathodecatalyst layer can be referred to as a first ion-conducting polymer.

The first ion-conducting polymer can comprise at least oneion-conducting polymer that is an anion-conductor. This can beadvantageous because it raises the pH compared to a proton conductor.

In some embodiments, the first ion-conducting polymer can comprise oneor more covalently-bound, positively-charged functional groupsconfigured to transport mobile negatively-charged ions. The firstion-conducting polymer can be selected from the group consisting ofaminated tetramethyl polyphenylene;poly(ethylene-co-tetrafluoroethylene)-based quaternary ammonium polymer;quaternized polysulfone), blends thereof, and/or any other suitableion-conducting polymers. The first ion-conducting polymer can beconfigured to solubilize salts of bicarbonate or hydroxide.

In some embodiments, the first ion-conducting polymer can comprise atleast one ion-conducting polymer that is a cation-and-anion-conductor.The first ion-conducting polymer can be selected from the groupconsisting of polyethers that can transport cations and anions andpolyesters that can transport cations and anions. The firstion-conducting polymer can be selected from the group consisting ofpolyethylene oxide, polyethylene glycol, polyvinylidene fluoride, andpolyurethane.

A cation-and-anion conductor will raise pH (compared to a pure cationconductor.) Further, in some embodiments, it may be advantageous to usea cation-and-anion conductor to promote acid base recombination in alarger volume instead of at a 2D interface of anion-conducting polymerand cation conducting polymer. This can spread out water and CO₂formation, heat generation, and potentially lower the resistance of themembrane by decreasing the barrier to the acid-base reaction. All ofthese may be advantageous in helping avoid the buildup of products,heat, and lowering resistive losses in the MEA leading to a lower cellvoltage.

A typical anion-conducting polymer has a polymer backbone withcovalently bound positively charged functional groups appended. Thesemay include positively charged nitrogen groups in some embodiments. Insome embodiments, the polymer backbone is non-polar, as described above.The polymer may be any appropriate molecular weight, e.g., 25,000g/mol-150,000 g/mol, though it will be understood that polymers outsidethis range may be used.

Particular challenges for ion-conducting polymers in CRR's include thatCO₂ can dissolve or solubilize polymer electrolytes, making them lessmechanically stable, prone to swelling, and allowing the polymer to movemore freely. This makes the entire catalyst layer andpolymer-electrolyte membrane less mechanically stable. In someembodiments, polymers that are not as susceptible to CO₂ plasticizationare used. Also, unlike for water electrolyzers and fuel cells,conducting carbonate and bicarbonate ions is a key parameter for CO₂reduction.

The introduction of polar functional groups, such as hydroxyl andcarboxyl groups which can form hydrogen bonds, leads topseudo-crosslinked network formation. Cross-linkers like ethylene glycoland aluminum acetylacetonate can be added to reinforce the anionexchange polymer layer and suppress polymer CO₂ plasticization.Additives like polydimethylsiloxane copolymer can also help mitigate CO₂plasticization.

According to various embodiments, the ion-conducting polymer may have abicarbonate ionic conductivity of at least 12 mS/cm, is chemically andmechanically stable at temperatures 80° C. and lower, and soluble inorganic solvents used during fabrication such as methanol, ethanol, andisoproponal. The ion-conducting polymer is stable (chemically and hasstable solubility) in the presence of the CO_(x) reduction products. Theion-conducting polymer may also be characterized by its ion exchangecapacity, the total of active sites or functional groups responsible forion exchange, which may range from 2.1 mmol/g-2.6 mmol/g in someembodiments.

Examples of anion-conducting polymers are given above in above table asClass A ion-conducting polymers. A particular example of ananion-conducting polymer is Orion mTPN1, which has m-triphenylfluori-alkylene as backbone and trimethylamonium (TMA+) as cation group.The chemical structure is shown below.

Additional examples include anion exchange membranes produced byFumatech and Ionomr. Fumatech FAA-3 ionomers come in Br— form. Anionexchange polymer/membrane based on polybenzimidazole produced by Ionomrcomes in I— form as AF-1-HNN8-50-X.

The as-received polymer may be prepared by exchanging the anion (e.g.,I⁻, Br⁻, etc.) with bicarbonate.

Also, as indicated above, in certain embodiments the ionomer may be acation-and-ion-conducting polymer. Examples are given in the above tableas Class B ion-conducting polymers.

Metal Catalyst (Cathode Catalyst Layer)

The metal catalyst catalyzes the COx reduction reaction(s). The metalcatalyst is typically nanoparticles, but larger particles, films, andnanostructured surfaces may be used in some embodiments. The specificmorphology of the nanoparticles may expose and stabilize active sitesthat have greater activity.

The metal catalyst is often composed of pure metals (e.g., Cu, Au, Ag),but specific alloys or other bimetallic systems may have high activityand be used for certain reactions. The choice of catalyst may be guidedby the desired reaction. For example, for CO production, Au may be used;for methane and ethylene production, Cu may be used. Other metalsincluding Ag, alloys, and bimetallic systems may be used. CO₂ reductionhas a high overpotential compared to other well-known electrochemicalreactions such as hydrogen evolution and oxygen evolution on knowncatalysts. Small amounts of contaminants can poison catalysts for CO₂conversion. And as indicated above, metal catalysts such as Cu, Au, andAg are less developed than catalysts such as platinum used in hydrogenfuel cells.

Metal catalyst properties that affect the cathode catalyst layerperformance include size, size distribution, uniformity of coverage onthe support particles, shape, loading (characterized as weight ofmetal/weight of metal+weight of carbon or as mass of particles pergeometric area of catalyst layer), surface area (actual metal catalystsurface area per volume of catalyst layer), purity, and the presence ofpoisoning surface ligands from synthesis.

Nanoparticles may be synthesized by any appropriate method, such as forexample, described in Phan et al., “Role of Capping Agent in WetSynthesis of Nanoparticles,” J. Phys. Chem. A 2018, 121, 17, 3213-3219;Bakshi “How Surfactants Control Crystal Growth of Nanomaterials,” Cryst.Growth Des. 2016, 16, 2, 1104-1133; and Morsy “Role of Surfactants inNanotechnology and Their Applications,” Int. J. Curr. Microbiol. App.Sci. 2014, 3, 5, 237-260, which are incorporated by reference herein.

In some embodiments, metal nanoparticles are provided without thepresence of poisoning surface ligands. This may be achieved by using theionomer as a ligand to direct the synthesis of nanocrystal catalysts.The surface of the metal nanocatalysts are directly connected withionically conductive ionomer. This avoids having to treat the catalystsurface to allow ionomer contact with the metal and improves thecontact.

The metal catalyst may be disposed on a carbon support in someembodiments. For CO production, examples include Premetek 20 wt % Ausupported on Vulcan XC-72R carbon with 4-6 nm Au particle size and 30%Au/C supported on Vulcan XC-72R with 5-7 nm Au particle size. Formethane, examples include Premetek 20 wt % Cu supported on Vulcan XC-72Rcarbon with 20-30 nm Cu particle size. In some embodiments, the metalcatalyst may be unsupported. For ethylene production, examples ofunsupported metal catalysts include SigmaAldrich unsupported Cu 80 nmparticle size and ebeam or sputter deposited thin Cu layer of 10 nm to100 nm.

Support (Cathode Catalyst Layer)

The support of the cathode catalyst layer may have various functions. Itmay stabilize metal nanoparticles to prevent them from agglomerating anddistributed the catalytic sites throughout the catalyst layer volume tospread out loss of reactants and formation of products. It may also forman electronically form an electrically conductive pathway to metalnanoparticles. Carbon particles, for example, pack together such thatcontacting carbon particles provide the electrically conductive pathway.Void space between the particles forms a porous network that gas andliquids can travel through.

In some embodiments, carbon supports developed for fuel cells can beused. Many different types have been developed; these are typically 50nm-500 nm in size, and can be obtained in different shapes (spheres,nanotubes, sheets (e.g., graphene)), porosities, surface area pervolume, electrical conductivity, functional groups (N-doped, O-doped,etc).

The support may be hydrophobic and have affinity to the metalnanoparticle.

Examples of carbon blacks that can be used include:

-   -   Vulcan XC-72R-Density of 256 mg/cm2, 30-50 nm    -   Ketjen Black-Hollow structure, Density of 100-120 mg/cm2, 30-50        nm    -   Printex Carbon, 20-30 nm

Anode Catalyst Layer

The anode of the MEA, which is also referred to as the anode layer oranode catalyst layer, facilitates oxidation reactions. It is a porouslayer containing catalysts for oxidation reactions. Examples ofreactions are:

2H₂O→4H⁺+4e ⁻+O₂ (in acidic environment of proton exchange polymerelectrolyte-bipolar membrane); or

4OH⁻→4e ⁻+O₂+2H₂O (in basic environment of anion exchange polymerelectrolyte)

The oxidation of other materials, such as hydrocarbons to make CO₂ orchloride ions to make chlorine gas, may also be performed.

In some embodiments, with reference to FIG. 2, the anode 240 contains ablend of oxidation catalyst and an anode ion-conducting polymer. Thereare a variety of oxidation reactions that can occur at the anodedepending on the reactant that is fed to the anode and the anodecatalyst(s). In one arrangement, the oxidation catalyst is selected fromthe group consisting of metals and oxides of Ir, Pt, Ni, Ru, Pd, Au, andalloys thereof, IrRu, PtIr, Ni, NiFe, stainless steel, and combinationsthereof. The oxidation catalyst can further contain conductive supportparticles selected from the group consisting of carbon, boron-dopeddiamond, and titanium.

The oxidation catalyst can be in the form of a structured mesh or can bein the form of particles. If the oxidation catalyst is in the form ofparticles, the particles can be supported by electronically-conductivesupport particles. The conductive support particles can benanoparticles. It is especially useful if the conductive supportparticles are compatible with the chemicals that are present in theanode 240 when the CRR is operating and are oxidatively stable so thatthey do not participate in any electrochemical reactions. It isespecially useful if the conductive support particles are chosen withthe voltage and the reactants at the anode in mind. In somearrangements, the conductive support particles are titanium, which iswell-suited for high voltages. In other arrangements, the conductivesupport particles are carbon, which can be most useful at low voltages.In general, such conductive support particles are larger than theoxidation catalyst particles, and each conductive support particle cansupport many oxidation catalyst particles. An example of such anarrangement is shown in FIG. 3 and is discussed above with respect tothe cathode catalyst layer. In one arrangement, the oxidation catalystis iridium ruthenium oxide. Examples of other materials that can be usedfor the oxidation catalyst include, but are not limited to, those listedabove. It should be understood that many of these metal catalysts can bein the form of oxides, especially under reaction conditions.

In some embodiments, the MEA has an anode layer comprising oxidationcatalyst and a second ion-conducting polymer. The second ion-conductingpolymer can comprise one or more polymers that contain covalently-bound,negatively-charged functional groups configured to transport mobilepositively-charged ions. The second ion-conducting polymer can beselected from the group consisting of ethanesulfonyl fluoride,2-[1-[difluoro-[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1,1,2,2,-tetrafluoro-,with tetrafluoroethylene,tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acidcopolymer, other perfluorosulfonic acid polymers and blends thereof.Examples of cation-conducting polymers include e.g., Nafion 115, Nafion117, and/or Nafion 211.

There are tradeoffs in choosing the amount of ion-conducting polymer inthe anode. It is important to include enough anode ion-conductingpolymer to provide sufficient ionic conductivity. But it is alsoimportant for the anode to be porous so that reactants and products canmove through it easily, and to maximize the amount of catalyst surfacearea that is available for reaction. In various arrangements, theion-conducting polymer in the anode makes up approximately 50 wt % ofthe layer or between approximately 5 and 20 wt %, 10 and 90 wt %,between 20 and 80 wt %, between 25 and 70 wt %, or any suitable range.It is especially useful if the anode 240 can tolerate high voltages,such as voltages above about 1.2 V vs. a reversible hydrogen electrode.It is especially useful if the anode 240 is porous in order to maximizethe amount of catalyst surface area available for reaction and tofacilitate gas and liquid transport.

In one example of a metal catalyst, Ir or IrOx particles (100-200 nm)and Nafion ionomer form a porous layer approximately 10 μm thick. Metalcatalyst loading is approximately 0.5-3 g/cm².

In some embodiments, NiFeO_(x) is used for basic reactions.

PEM

The MEAs include a polymer electrolyte membrane (PEM) disposed betweenand conductively coupled to the anode catalyst layer and the cathodecatalyst layer. Referring to FIG. 2, the polymer electrolyte membrane265 has high ionic conductivity (greater than about 1 mS/cm), and ismechanically stable. Mechanical stability can be evidenced in a varietyof ways such as through high tensile strength, modulus of elasticity,elongation to break, and tear resistance. Many commercially-availablemembranes can be used for the polymer electrolyte membrane 265. Examplesinclude, but are not limited to, various Nafion® formulations,GORE-SELECT, FumaPEM® (PFSA) (FuMA-Tech GmbH), and Aquivion (PFSA)(Solvay).

In one arrangement, the PEM comprises at least one ion-conductingpolymer that is a cation-conductor. The third ion-conducting polymer cancomprise one or more covalently-bound, negatively-charged functionalgroups configured to transport mobile positively-charged ions. The thirdion-conducting polymer can be selected from the group consisting ofethanesulfonyl fluoride,2-[1-[difluoro-[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1,1,2,2,-tetrafluoro-,with tetrafluoroethylene,tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acidcopolymer, other perfluorosulfonic acid polymers and blends thereof.

Cathode Buffer Layer

Referring to FIG. 2, it may be noted that when the polymer electrolytemembrane 265 is a cation conductor and is conducting protons, itcontains a high concentration of protons during operation of the CRR,while the cathode 220 operates best when a low concentration of protonsis present. It can be useful to include a cathode buffer layer 225between the polymer electrolyte membrane 265 and the cathode 220 toprovide a region of transition from a high concentration of protons to alow concentration of protons. In one arrangement, the cathode bufferlayer 225 is an ion-conducting polymer with many of the same propertiesas the ion-conducting polymer in the cathode 220. The cathode bufferlayer 225 provides a region for the proton concentration to transitionfrom the polymer electrolyte membrane 265, which has a highconcentration of protons to the cathode 220, which has a low protonconcentration. Within the cathode buffer layer 225, protons from thepolymer electrolyte membrane 265 encounter anions from the cathode 220,and they neutralize one another. The cathode buffer layer 225 helpsensure that a deleterious number of protons from the polymer electrolytemembrane 265 does not reach the cathode 220 and raise the protonconcentration. If the proton concentration of the cathode 220 is toohigh, COx reduction does not occur. High proton concentration isconsidered to be in the range of approximately 10 to 0.1 molar and lowconcentration is considered to be less than approximately 0.01 molar.

The cathode buffer layer 225 can include a single polymer or multiplepolymers. If the cathode buffer layer 225 includes multiple polymers,the multiple polymers can be mixed together or can be arranged inseparate, adjacent layers. Examples of materials that can be used forthe cathode buffer layer 225 include, but are not limited to, FumaSepFAA-3, Tokuyama anion exchange membrane material, and polyether-basedpolymers, such as polyethylene oxide (PEO), and blends thereof. Furtherexamples are given above in the discussion of the cathode catalystlayer.

The thickness of the cathode buffer layer is chosen to be sufficientthat COx reduction activity is high due to the proton concentrationbeing low. This sufficiency can be different for different cathodebuffer layer materials. In general, the thickness of the cathode bufferlayer is between approximately 200 nm and 100 between 300 nm and 75between 500 nm and 50 or any suitable range.

In some embodiments, the cathode buffer layer is less than 50 μm, forexample between 1-25 μm such between 1-5 μm, 5-15 μm, or 10-25 μm. Byusing a cathode buffer layer in this range of thicknesses, the protonconcentration in the cathode can be reduced while maintaining theoverall conductivity of the cell. In some embodiments, an ultra-thinlayer (100 nm-1 μm and in some embodiments, sub-micron) may be used. Andas discussed above, in some embodiments, the MEA does not have a cathodebuffer layer. In some such embodiments, anion-conducting polymer in thecathode catalyst layer is sufficient. The thickness of the cathodebuffer layer may be characterized relative to that of the PEM.

Water and CO₂ formed at the interface of a cathode buffer layer and aPEM can delaminate the MEA where the polymer layers connect. Thedelamination problem can be addressed by employing a cathode bufferlayer having inert filler particles and associated pores. One possibleexplanation of its effectiveness is that the pores create paths for thegaseous carbon dioxide to escape back to the cathode where it can bereduced.

Materials that are suitable as inert filler particles include, but arenot limited to, TiO₂, silica, PTFE, zirconia, and alumina. In variousarrangements, the size of the inert filler particles is between 5 nm and500 μm, between 10 nm and 100 μm, or any suitable size range. Theparticles may be generally spherical.

If PTFE (or other filler) volume is too high, it will dilute the polymerelectrolyte to the point where ionic conductivity is low. Too muchpolymer electrolyte volume will dilute the PTFE to the point where itdoes not help with porosity. In many embodiments a mass ratio of polymerelectrolyte/PTFE is 0.25 to 2, and more particularly, 0.5 to 1. A volumeratio polymer electrolyte/PTFE (or, more generally, polymerelectrolyte/inert filler) may be 0.25 to 3, 0.5 to 2, 0.75 to 1.5, or1.0 to 1.5.

In other arrangements, porosity is achieved by using particularprocessing methods when the layers are formed. One example of such aprocessing method is laser ablation, where nano to micro-sized channelsare formed in the layers. Another example is mechanically puncturing alayer to form channels through it.

In one arrangement, the cathode buffer layer has a porosity between0.01% and 95% (e.g., approximately between, by weight, by volume, bymass, etc.). However, in other arrangements, the cathode buffer layercan have any suitable porosity (e.g., between 0.01-95%, 0.1-95%,0.01-75%, 1-95%, 1-90%). In some embodiments, the porosity is 50% orless, e.g., 0.1-50%, 5-50%, 20-50%, 5-40%, 10-40%, 20-40%, or 25%-40%.In some embodiments, the porosity is 20% or below, e.g. 0.1-20%, 1-10%,or 5-10%.

Porosity may be measured as described above with respect to the catalystlayer, including using mass loadings and thicknesses of the components,by methods such as mercury porosimetry, x-ray diffraction (SAXS orWAXS), and image processing on TEM images to calculate filled space vs.empty space. Porosity is measured when the MEA is completely dry as thematerials swell to varying degrees when exposed to water duringoperation.

Porosity in layers of the MEA, including the cathode buffer layer, isdescribed further below.

Anode Buffer Layer

In some CRR reactions, bicarbonate is produced at the cathode 220. Itcan be useful if there is a polymer that blocks bicarbonate transportsomewhere between the cathode 220 and the anode 240, to preventmigration of bicarbonate away from the cathode. It can be thatbicarbonate takes some CO₂ with it as it migrates, which decreases theamount of CO₂ available for reaction at the cathode. In one arrangement,the polymer electrolyte membrane 265 includes a polymer that blocksbicarbonate transport. Examples of such polymers include, but are notlimited to, Nafion® formulations, GORE-SELECT, FumaPEM® (PFSA)(FuMA-Tech GmbH), and Aquivion (PFSA) (Solvay). In another arrangement,there is an anode buffer layer 245 between the polymer electrolytemembrane 265 and the anode 240, which blocks transport of bicarbonate.If the polymer electrolyte membrane is an anion-conductor, or does notblock bicarbonate transport, then an additional anode buffer layer toprevent bicarbonate transport can be useful. Materials that can be usedto block bicarbonate transport include, but are not limited to Nafion®formulations, GORE-SELECT, FumaPEM® (PFSA) (FuMA-Tech GmbH), andAquivion (PFSA) (Solvay). Of course, including a bicarbonate blockingfeature in the ion-exchange layer 260 is not particularly desirable ifthere is no bicarbonate in the CRR.

In another embodiment of the invention, the anode buffer layer 245provides a region for proton concentration to transition between thepolymer electrolyte membrane 265 to the anode 240. The concentration ofprotons in the polymer electrolyte membrane 265 depends both on itscomposition and the ion it is conducting. For example, a Nafion polymerelectrolyte membrane 265 conducting protons has a high protonconcentration. A FumaSep FAA-3 polymer electrolyte membrane 265conducting hydroxide has a low proton concentration. For example, if thedesired proton concentration at the anode 240 is more than 3 orders ofmagnitude different from the polymer electrolyte membrane 265, then ananode buffer layer 245 can be useful to effect the transition from theproton concentration of the polymer electrolyte membrane 265 to thedesired proton concentration of the anode. The anode buffer layer 245can include a single polymer or multiple polymers. If the anode bufferlayer 245 includes multiple polymers, the multiple polymers can be mixedtogether or can be arranged in separate, adjacent layers. Materials thatcan be useful in providing a region for the pH transition include, butare not limited to, Nafion, FumaSep FAA-3, Sustainion®, Tokuyama anionexchange polymer, and polyether-based polymers, such as polyethyleneoxide (PEO), blends thereof, and/or any other suitable materials. Highproton concentration is considered to be in the range of approximately10 to 0.1 molar and low concentration is considered to be less thanapproximately 0.01 molar. Ion-conducting polymers can be placed indifferent classes based on the type(s) of ions they conduct. This hasbeen discussed in more detail above. There are three classes ofion-conducting polymers described in Table 4 above. In one embodiment ofthe invention, at least one of the ion-conducting polymers in thecathode 220, anode 240, polymer electrolyte membrane 265, cathode bufferlayer 225, and anode buffer layer 245 is from a class that is differentfrom at least one of the others.

Layer Porosity

It can be useful if some or all of the following layers are porous: thecathode 220, the cathode buffer layer 225, the anode 240 and the anodebuffer layer 245. In some arrangements, porosity is achieved bycombining inert filler particles with the polymers in these layers.Materials that are suitable as inert filler particles include, but arenot limited to, TiO₂, silica, PTFE, zirconia, and alumina. In variousarrangements, the size of the inert filler particles is between 5 nm and500 μm, between 10 nm and 100 μm, or any suitable size range. In otherarrangements, porosity is achieved by using particular processingmethods when the layers are formed. One example of such a processingmethod is laser ablation, where nano to micro-sized channels are formedin the layers. Laser ablation can additionally or alternatively achieveporosity in a layer by subsurface ablation. Subsurface ablation can formvoids within a layer, upon focusing the beam at a point within thelayer, and thereby vaporizing the layer material in the vicinity of thepoint. This process can be repeated to form voids throughout the layer,and thereby achieving porosity in the layer. The volume of a void ispreferably determined by the laser power (e.g., higher laser powercorresponds to a greater void volume), but can additionally oralternatively be determined by the focal size of the beam, or any othersuitable laser parameter. Another example is mechanically puncturing alayer to form channels through the layer. The porosity can have anysuitable distribution in the layer (e.g., uniform, an increasingporosity gradient through the layer, a random porosity gradient, adecreasing porosity gradient through the layer, a periodic porosity,etc.).

The porosities (e.g., of the cathode buffer layer, of the anode bufferlayer, of the membrane layer, of the cathode layer, of the anode layer,of other suitable layers, etc.) of the examples described above andother examples and variations preferably have a uniform distribution,but can additionally or alternatively have any suitable distribution(e.g., a randomized distribution, an increasing gradient of pore sizethrough or across the layer, a decreasing gradient of pore size throughor across the layer, etc.). The porosity can be formed by any suitablemechanism, such as inert filler particles (e.g., diamond particles,boron-doped diamond particles, polyvinylidene difluoride/PVDF particles,polytetrafluoroethylene/PTFE particles, etc.) and any other suitablemechanism for forming substantially non-reactive regions within apolymer layer. The inert filler particles can have any suitable size,such as a minimum of about 10 nanometers and a maximum of about 200nanometers, and/or any other suitable dimension or distribution ofdimensions.

As discussed above, the cathode buffer layer preferably has a porositybetween about 1 and 90 percent by volume, but can additionally oralternatively have any suitable porosity (including, e.g., no porosity).However, in other arrangements and examples, the cathode buffer layercan have any suitable porosity (e.g., between 0.01-95%, 0.1-95%,0.01-75%, 1-95%, 1-90%, etc.). in some embodiments, the porosity is 20%or below, e.g. 0.1-20%, 1-10%, or 5-10%.

In some embodiments, the cathode buffer layer is porous but at least onelayer between the cathode layer and the anode layer is nonporous. Thiscan prevent the passage of gases and/or bulk liquid between the cathodeand anode layers while still preventing delamination. For example, thenonporous layer can prevent the direct passage of water from the anodeto the cathode.

MEA Fabrication

MEAs for CO_(x) reduction may be fabricated using a variety oftechniques. In various embodiments, MEAs fabrication employs multiplesteps. Small differences in the parameters of the fabrication processcan make a large difference in performance.

In certain embodiments, MEA fabrication employs a polymer-electrolytemembrane (e.g., a Nafion PEM) layer and depositing or otherwise formingan anion-exchange polymer electrolyte layer and cathode catalyst layeron the cathode and depositing or otherwise forming an anode catalystlayer on the anode. An alternate route is to fabricate the catalystlayers on to porous gas diffusion layers (e.g., carbon for the cathodeor titanium for the anode) and sandwich the membrane (which may includethe anion-exchange layer) between catalyst containing porous layers. Incertain embodiments, catalyst layers are fabricated by making an ink ofthe solid catalyst and support particles and polymer electrolytedispersed in a solvent. The ink may be applied by a variety of methodsto the polymer electrolyte membrane or GDL. The solvent subsequentlyevaporates leaving behind a porous solid catalyst layer.

Imaging methods may be used to characterize the thickness anduniformity. The thickness should be consistent and controllable, and theuniformity smooth and as defect free as possible.

Various techniques may be employed to form the individual layers of theMEA. Generally, these techniques form the layer on a substrate such as aPEM layer or GDL as mentioned herein. Examples of such techniquesinclude ultrasonic spray deposition, doctor blade application, gravure,screen printing, and decal transfer

Catalyst inks using anion-exchange polymers are not well studied(particularly for certain polymers) and do not have the same solutionstructure as typical Nafion-based inks used in fuel cells andelectrolyzers. The formulation and steps needed for form a welldispersed and stable catalyst ink were not known. It is believed thatNafion forms micell-like structures that allow relatively easysuspension in aqueous media. Other ion-conducting polymers andparticularly some anion-conducting polymers do not form such structuresand therefore are more difficult to provide in suspensions.

In certain embodiments, a catalyst layer ink is prepared by mixing metalor metal supported on carbon catalyst with ion-conducting polymer (e.g.,an anion-conducting polymer) and dispersing in solvent (alcohol, etc.)by sonicating.

As indicated, certain fabrication techniques utilize doctor bladeapplication, screen printing, decal transfer, electrospinning, etc.Roll-to-roll techniques such as gravure or microgravure may be used forhigh throughput processing.

MEA Post Treatments

After the MEA is fabricated, additional treatments may be used toincrease performance. Examples the types of performance improvementinclude lifetime and voltage. In some embodiments, a post treatmentintroduces salt or certain salt ions into an MEA. In some embodiments, apost treatment produces an MEA that has structural modificationsresulting from the treatments including better adhesion between layers.

Hot pressing: heating the MEA under pressure to bond the layerstogether. Hot pressing will help ‘melt’ layers together to preventdelamination.

-   -   Time: about 2 min to 10 min (MEA only); 1.5 min-2 min (MEA+gas        distribution layer (GDL)); the “MEA+GDL” may be pressed at least        twice to form a stable assembly    -   Temperature: about 100° C. to 150° C.;    -   Pressure: between about 300 psi and 600 psi (for 3×3 inch ½        MEAs), but the MEA can tolerate about 2500 psi without GDL;

Hydration: soaking the MEA in water or aqueous solutions to wet thepolymer-electrolytes prior to cell assembly. In some embodiments, theaqueous solution is a salt solution as described herein.

Boil Nafion or other polymer electrolyte MEA. This permanently changesthe macrostructure of the polymer electrolyte and increases the amountof water in the polymer matrix. This increases ionic conductivity, butalso increases water transport number.

Heat to dry. This can decrease water content and can reduce the amountof water transported through the polymer electrolyte during operation.

Stabilized Interface Between MEA Layers

Water and CO₂ formed at the interface of an anion-conducting layer(e.g., a cathode buffer layer) and a cation-conducting membrane (e.g., aPEM) can cause the two layers to separate or delaminate where thepolymer layers connect. The reaction at the bipolar interface isdepicted in FIGS. 3 and 7.

In addition, it is desirable for the CO₂ to return to the cathode of thecell where it can be reduced instead of lost to the anode, so a pathway(e.g., pores) in an anion-exchange layer (e.g., a cathode buffer layerand/or cathode layer) provides both a way to remove water and CO₂ fromthe interface to prevent delamination and return CO₂ to the cathodewhere it can react.

The structure depicted in FIG. 7 is similar to that depicted in FIG. 3,but FIG. 7 includes additional information relevant to mass transportand generation of CO₂ and water at a bipolar interface. For example, itshows hydroxide and CO₂ reacting on the cathode side to producebicarbonate ions, which move toward the bipolar interface 713. On theanode side, hydrogen ions produced by water oxidation move towardbipolar interface 713, where they react with the bicarbonate ions toproduce water and CO₂, both of which should be allowed to escape withoutdamaging the bipolar layers.

Also depicted in FIG. 7 are water transport paths including (a)electroosmotic drag with anions from the cathode to interface 713, (b)electroosmotic drag with cations from the anode to interface 713, and(c) diffusion. Water evaporates at the anode and cathode.

Various MEA designs contain features that resist delamination andoptionally provide a pathway for the reaction products to leave theinterface area. In some embodiments, the bipolar interface is flat. Butin some designs, the interface is provided with a composition gradientand/or interlocking structures. These are described further below withreference to FIGS. 10a, 10b, 10c , and 10 d, which illustrate bipolarinterfaces of MEA designs configured to resist delamination.

In some embodiments, the interface includes a gradient. A gradient maybe formed, for example, by using two nozzles during spray deposition andadding anion-exchange polymer with the relative amounts of the polymersvaried during deposition of the cation-exchange layer. Similarly,cation-exchange polymer may be added during deposition of theanion-exchange layer. Referring for example to FIG. 7, a gradient mayextend through substantially all or a portion of the anion-exchangeregion and cation-exchange region, such that the anion-exchange regionhas predominantly anion-exchange polymer adjacent to the cathode withthe relative amount of cation-exchange polymer increasing moving fromthe cathode toward the interface 713. Similarly, the cathode-exchangeregion has a predominantly cation-exchange polymer adjacent the anodecathode with the relative amount of anion-exchange polymer increasingmoving from the anode toward the interface 713. In some embodiments,there are a pure anion-exchange and pure cation-exchange regions with agradient between the two.

In some embodiments, the layers of the bipolar membrane are meltedtogether. This may be accomplished by choosing an appropriate solvent.For example, Nafion is at least slightly soluble in a water/ethanolmixture. By using that mixture (or another solvent in which thecation-conducting polymer is soluble) as a solvent for theanion-conducting polymer can result in Nafion or other cation-conductingpolymer at least slightly dissolvent and melting into the interface. Insome embodiments, this results in a thin gradient, e.g., one thatextends 0.5-10% into the anion-conducting polymer layer thickness.

In some embodiments, the interface includes a mixture of the polymers.FIG. 8A illustrates a bipolar interface 813 in which a cation-conductingpolymer 821 and an anion-conducting polymer 819 are mixed. In theexample of FIG. 8A, a portion of an anion-conducting polymer layer 809and a portion of a cation-conducting polymer layer 811 are shown. Theanion-conducting polymer layer 809 may be a pure anion-conductingpolymer and the cation-conducting polymer layer 811 may be pure cationexchange polymer. The cation-conducting polymer 821 may be the same ordifferent cation-conducting polymer as in the cation-conducting polymerlayer 811. The anion-conducting polymer 819 may be the same or differentanion-conducting polymer as in the anion-conducting polymer layer 809.

In some embodiments, the interface includes a third material thatphysically reinforces the interface. For example, FIG. 8B shows anexample of a material 830 that straddles interface 813. That is, thematerial 830 partially resides in an anion-conducting polymer layer 809and a cation-conducting polymer layer 811. Because of this, material 830may bind the two layers in a manner that resists delamination. In oneexample, the material 830 is a porous inert material, such as porousPTFE. Such an interface may be fabricated, for example, by casting orotherwise applying the cation-conducting polymer and theanion-conducting polymer on opposite sides of a PTFE or similar porousfilm, followed by hot pressing.

FIG. 8C illustrates a bipolar interface 813 having protrusions 840 ofthe cation-conducting polymer extending from the cation-conductingpolymer layer 811 into the anion-conducting polymer layer 809. Theseprotrusions may mechanically strengthen interface 813 so that it doesnot delaminate when CO₂ and water are produced at the interface. In someembodiments, protrusions extend from anion-conducting polymer layer 809into cation-conducting polymer layer 811. In certain embodiments,protrusions extend both directions. Example dimensions are 10 μm-1 mm inthe in-plane dimension, though smaller dimensions (e.g., 500 nm-1 μm)are possible. The out-of-plane dimension may be for example, 10-75% or10-50% of the total thickness of the polymer layer into which itextends. The protrusions may be fabricated for example by anyappropriate technique such as lithographic techniques or by spraying thepolymer into a patterned mesh that is then removed. Surface rougheningtechniques may also be used to create protrusions. In some embodiments,protrusions may be formed from a different material, e.g., metal to helpinterlock the polymer layers and mechanically strengthen the interface.

FIG. 8D illustrates a bipolar interface 813 having a third material 850disposed between or mixed one or more of the cation-conducting polymerlayer 811 into the anion-conducting polymer layer 809. In someembodiments, for example, the third material 850 can be an additive asdiscussed further below. In some embodiments, the third material 850 canbe a blend of anion-conducting and cation-conducting ionomers at theinterface. For example, it can be a mixture of Nafion 5 wt % ionomer andOrion 2 wt % mTPN1. In some embodiments, the third material may includeion acceptors and donors, either mixed together or provided as distinctlayers.

In some embodiments, the interface includes additives to facilitateacid-base reactions and prevent delamination. In some embodiments, theadditives may facilitate spreading out the acid base recombination alarger volume instead of just at a 2D interface of the anion conductingpolymer and cation conducting polymer. This spreads out water and CO₂formation, heat generation, and may lower the resistance of the membraneby decreasing the barrier to the acid-base reaction. These effects canbe advantageous in helping avoid build-up of products, heat, andlowering resistive losses in the MEA leading to a lower cell voltage.Further, it helps avoid degrading materials at the interface due to heatand gas production.

Examples of additives that facilitate acid-base reactions includemolecules that are both proton and anion acceptors, such as hydroxidecontaining ionic liquids with 1-butyl-3-methylimidazolium hydroxidebeing a specific example. Other ionic liquids may also be used. In someembodiments, an ionomer different from that of the anion-conductivepolymer layer and the cation-conductive polymer layer may be used. Forexample, a relatively high conductivity anion-exchange material such asSustainion may be used. Such anion-exchange material may not beselective enough to use as a cathode buffer layer, but can be used atthe interface.

Additional examples of materials that may be present at the interfaceinclude block copolymers having different charged groups (e.g., bothcation and anion stationary charge groups), cation-and-anion conductingpolymers, resin material, ion donors such as oxides including grapheneoxide, catalysts for acid/base recombination, catalysts that react H₂and O₂ diffusing from the anode and cathode, water splitting catalysts,CO₂ absorbing material, and H₂ absorbing material.

In some embodiments, a cross-linker may be added to covalentlycross-link the two polymers of the bipolar membrane. Examples ofcross-linking groups include xylene, which may be provided on anionomer. Other cross-linking groups may be used. A cross-linker may beprovided, for example, on the cation-conductive polymer, with theanion-conductive polymer spray-deposited on top, followed by heating toinduce the cross-linking reaction and introduce cross-linking across theinterface.

In some embodiments, the anion-conducting polymer and thecation-conducting polymer of the bipolar membrane have the samebackbone, with different stationary charge groups. As an example, Orionionomers may be used with different stationary charge groups. Theionomers are more compatible and less apt to delaminate.

In the examples above, the interface 813 may be a three-dimensionalvolume having thickness that is between 1% and 90% of the overallthickness of the bipolar membrane, or between 5% and 90%, or between 10%and 80%, or between 20% and 70%, or between 30% and 60% of the overallthickness of the bipolar membrane. In some embodiments, it less thanhalf the overall thickness, including between 1% and 45%, 5% and 45%, 5%and 40%, or 5% and 30%.

Hot pressing may be used in fabricating any of the bipolar interfacedesigns described above.

Relative Sizes of MEA Layers

In certain embodiments, a polymer electrolyte membrane and an adjoiningcathode buffer layer or other anion-conducting polymer layer may haverelative thickness that facilitate the fabrication and/or operatingperformance of an MEA.

FIG. 9 depicts an example of a partial MEA that includes ananion-conducting polymer layer (AEM) 903, which may be a cathode bufferlayer, and a polymer electrolyte membrane (PEM) 905, which may becation-conducting polymer layer (e.g., a proton exchange polymer layer)or an anion-conducting polymer layer. In this example, the PEM 905 isrelatively thicker than the anion-conducting polymer layer 903, whichmay be a cathode buffer layer, and a polymer electrolyte membrane (PEM)905, which may be cation-conducting polymer layer (e.g., a protonexchange polymer layer) or an anion-conducting polymer layer. In thisexample, the PEM 905 is relatively thicker than the anion-conductingpolymer layer 903. For example, the PEM 905 may be 120 micrometerscompared with about 10-20 micrometers thick for the AEM 903.

In some cases, anion-conducting polymers such as those used inanion-conducting polymer layer 903 are substantially less conductivethan cation-conducting polymers such as those used in PEM 905.Therefore, to provide the benefits of a cathode buffer layer (e.g.,anion-conducting polymer layer 903) without substantially increasing theoverall resistance of the MEA, a relatively thin cathode buffer is used.However, when a cathode buffer layer becomes too thin, it becomesdifficult to handle during fabrication of the MEA and in other contexts.Therefore, in certain embodiments, a thin cathode buffer layer isfabricated on top of a relatively thicker PEM layer such as acation-conducting polymer layer. The anion-conducting polymer layer maybe fabricated on the PEM layer using, for example, any of thefabrication techniques described elsewhere herein.

In various embodiments, the polymer electrolyte membrane layer isbetween about 20 and 200 micrometers thick. In some embodiments, thepolymer electrolyte membrane layer is between about 60 and 120micrometers thick. In some embodiments, a thin polymer electrolytemembrane layer is used, being between about 20 and 60 micrometers thick.In some embodiments, a relatively thick polymer electrolyte layer isused, between about 120 and 200 micrometers thick.

In some embodiments, a thinner cathode buffer layer is used with athinner polymer electrolyte membrane. This can facilitate movement ofthe CO₂ formed at the interface back to cathode, rather than to theanode. In some embodiments, a thicker cathode buffer layer is used witha thicker polymer electrolyte membrane. This can result in reducing cellvoltage in some embodiments.

Factors that can influence the thickness of a cathode buffer layerinclude the ion selectivity of the anion-conducting polymer, theporosity of the anion-conducting polymer, the conformality of theanion-conducting polymer coating the polymer electrolyte membrane.

Many anion-conducting polymers are in the range of 95% selective foranions, with about 5% of the current being cations. Higher selectivityanion-conducting polymers, with greater than 99% selectivity for anionscan allow for a reduction in a significant reduction in thickness whileproviding a sufficient buffer.

Mechanical strength of an anion-conducting layer can also influence itsthickness, with stronger layers enabling thinner layers. Reducingporosity of an anion-conducting polymer may reduce the thickness of theanion-conducting layer.

In some implementations, a cathode buffer layer or otheranion-conducting polymer layer that abuts the polymer electrolytemembrane is between about 10 and 20 micrometers thick. Using a >99%selective polymer can allow the cathode buffer layer to be reduced tobetween 2 and 10 microns in some embodiments.

In some cases, the ratio of thicknesses of the polymer electrolytemembrane and the adjoining anion-conducting polymer layer is betweenabout 3:1-90:1 with the ratios at the higher end used with highlyselective anion-conducting polymer layers. In some embodiments, theratio is about 2:1-13:1, about 3:1-13.1, or about 7:1-13.1.

In certain embodiments, a relatively thinner PEM improves some aspectsof the MEA's performance. Referring to FIG. 9, for example, polymerelectrolyte membrane 905 may have a thickness of about 50 micrometers,while the anion-conducting layer may have a thickness between about 10and 20 micrometers. A thin PEM favors movement of water generated at theAEM/PEM interface to move toward the anode. The pressure of gas on thecathode side of the cell can be about 80-450 psi, which causes the waterat the interface to move to the anode. However, in some instances, athick PEM can cause the majority of water to move through the AEM to thecathode, which leads to flooding. By using a thin PEM, flooding can beavoided.

CO_(x) Reduction Reactor (CRR)

FIG. 10 is a schematic drawing that shows the major components of aCO_(x) reduction reactor (CRR) 1005, according to an embodiment of thedisclosure. The CRR 1005 has a membrane electrode assembly 1000 such asany of those described elsewhere herein. The membrane electrode assembly1000 has a cathode 1020 and an anode 1040, separated by an ion-exchangelayer 1060. The ion-exchange layer 1060 may include sublayers. Thedepicted embodiment has three sublayers: a cathode buffer layer 1025, apolymer electrolyte membrane 1065, and an optional anode buffer layer1045. In addition, the CRR 1005 has a cathode support structure 1022adjacent to the cathode 1020 and an anode support structure 1042adjacent to the anode 1040.

The cathode support structure 1022 has a cathode polar plate 1024, madeof, for example, graphite, to which a voltage can be applied. There canbe flow field channels, such as serpentine channels, cut into the insidesurfaces of the cathode polar plate 1024. There is also a cathode gasdiffusion layer 1026 adjacent to the inside surface of the cathode polarplate 1024. In some arrangements, there is more than one cathode gasdiffusion layer (not shown). The cathode gas diffusion layer 1026facilitates the flow of gas into and out of the membrane electrodeassembly 1000. An example of a cathode gas diffusion layer 1026 is acarbon paper that has a carbon microporous layer.

The anode support structure 1042 has an anode polar plate 1044, usuallymade of metal, to which a voltage can be applied. There can be flowfield channels, such as serpentine channels, cut into the insidesurfaces of the anode polar plate 1044. There is also an anode gasdiffusion layer 1046 adjacent to the inside surface of the anode polarplate 1044. In some arrangements, there is more than one anode gasdiffusion layer (not shown). The anode gas diffusion layer 1046facilitates the flow of gas into and out of the membrane electrodeassembly 1000. An example of an anode gas diffusion layer 1046 is atitanium mesh or titanium felt. In some arrangements, the gas diffusionlayers 1026, 1046 are microporous.

There are also inlets and outlets (not shown) associated with thesupport structures 1022, 1042, which allow flow of reactants andproducts, respectively, to the membrane electrode assembly 1000. Thereare also various gaskets (not shown) that prevent leakage of reactantsand products from the cell.

In one embodiment, a direct current (DC) voltage is applied to themembrane electrode assembly 1000 through the cathode polar plate 1024and the anode polar plate 1042. Water is supplied to the anode 1040 andis oxidized over an oxidation catalyst to form molecular oxygen (O2),releasing protons (H+) and electrons (e−). The protons migrate throughthe ion-exchange layer 1060 toward the cathode 1020. The electrons flowthrough an external circuit (not shown). In one embodiment, the reactionis described as follows:

2H₂O→4H⁺+4e ⁺+O₂

In other embodiments, other reactants can be supplied to the anode 1040and other reactions can occur.

While the depicted embodiment shows an ion-exchange layer having threesublayers, certain embodiments employ ion-exchange layers having only asingle layer (e.g., a cation conducting polymer layer or an anionconducting polymer layer). Other embodiments have only two sublayers.

The flow of reactants, products, ions, and electrons through a CRR 1105reactor is indicated in FIG. 11, according to an embodiment. The CRR1105 has a membrane electrode assembly 1100 such as any of the MEAsdescribed elsewhere herein. The membrane electrode assembly 1100 has acathode 1120 and an anode 1140, separated by an ion-exchange layer 1160.In certain embodiments, the ion-exchange layer 1160 has three sublayers:a cathode buffer layer 1125, a polymer electrolyte membrane 1165, and anoptional anode buffer layer 1145. In addition, the CRR 1105 has acathode support structure 1122 adjacent to the cathode 1120 and an anodesupport structure 1142 adjacent to the anode 1140.

The cathode support structure 1122 has a cathode polar plate 1124, whichmay be made of graphite, to which a voltage can be applied. There can beflow field channels, such as serpentine channels, cut into the insidesurfaces of the cathode polar plate 1124. There is also a cathode gasdiffusion layer 1126 adjacent to the inside surface of the cathode polarplate 1124. In some arrangements, there is more than one cathode gasdiffusion layer (not shown). The cathode gas diffusion layer 1126facilitates the flow of gas into and out of the membrane electrodeassembly 1100. An example of a cathode gas diffusion layer 1126 is acarbon paper that has a carbon microporous layer.

The anode support structure 1142 has an anode polar plate 1144, whichmay be made of metal, to which a voltage can be applied. There can beflow field channels, such as serpentine channels, cut into the insidesurfaces of the anode polar plate 1144. There is also an anode gasdiffusion layer 1146 adjacent to the inside surface of the anode polarplate 1144. In some arrangements, there is more than one anode gasdiffusion layer (not shown). The anode gas diffusion layer 1146facilitates the flow of gas into and out of the membrane electrodeassembly 1100. An example of an anode gas diffusion layer 1146 is atitanium mesh or titanium felt. In some arrangements, the gas diffusionlayers 1126, 1146 are microporous.

There can also be inlets and outlets associated with the supportstructures 1122, 1142, which allow flow of reactants and products,respectively, to the membrane electrode assembly 1100. There can also bevarious gaskets that prevent leakage of reactants and products from thecell.

CO_(x) can be supplied to the cathode 1120 and reduced over CO_(x)reduction catalysts in the presence of protons and electrons. The CO_(x)can be supplied to the cathode 1120 at pressures between 0 psig and 1000psig or any other suitable range. The CO_(x) can be supplied to thecathode 1120 in concentrations below 100% or any other suitablepercentage along with a mixture of other gases. In some arrangements,the concentration of CO_(x) can be as low as approximately 0.5%, as lowas 5%, or as low as 20% or any other suitable percentage.

In one embodiment, between approximately 10% and 100% of unreactedCO_(x) is collected at an outlet adjacent to the cathode 1120, separatedfrom reduction reaction products, and then recycled back to an inletadjacent to the cathode 1120. In one embodiment, the oxidation productsat the anode 1140 are compressed to pressures between 0 psig and 1500psig.

In one embodiment, multiple CRRs (such as the one shown in FIG. 10) arearranged in an electrochemical stack and are operated together. The CRRsthat make up the individual electrochemical cells of the stack can beconnected electrically in series or in parallel. Reactants are suppliedto individual CRRs and reaction products are then collected.

In accordance with some embodiments, inputs and outputs to the reactorare shown in FIG. 12. CO_(x) anode feed material, and electricity arefed to the reactor. CO_(x) reduction product and any unreacted CO_(x)leave the reactor. Unreacted CO_(x) can be separated from the reductionproduct and recycled back to the input side of the reactor. Anodeoxidation product and any unreacted anode feed material leave thereactor in a separate stream. Unreacted anode feed material can berecycled back to the input side of the reactor.

Various catalysts in the cathode of a CRR cause different products ormixtures of products to form from CO_(x) reduction reactions. Examplesof possible CO. reduction reactions at the cathode are described asfollows:

CO₂+2H⁺+2e ⁺→CO+H₂O

2CO₂+12H⁺+12e ⁻→CH₂CH₂+4H₂O

2CO₂+12H⁺+12e ⁻→CH₃CH₂OH+3H₂O

CO₂+8H⁻+8e ⁻→CH₄+2H₂O

2CO+8H⁺+8e ⁻→CH₂CH₂+2H₂O

2CO+8H⁻+8e ⁻→CH₃CH₂OH+H₂O

CO+6H⁺+8e ⁻→CH₄+H₂O

In some embodiment, a method of operating a CO_(x) reduction reactor, asdescribed in the embodiments above, involves applying a DC voltage tothe cathode polar plate and the anode polar plate, supplying oxidationreactants to the anode and allowing oxidation reactions to occur,supplying reduction reactants to the cathode and allowing reductionreactions to occur, collecting oxidation reaction products from theanode; and collecting reduction reaction products from the cathode.

In one arrangement, the DC voltage is greater than about −1.2V. Invarious arrangements, the oxidation reactants can be any of hydrogen,methane, ammonia, water, or combinations thereof, and/or any othersuitable oxidation reactants. In one arrangement, the oxidation reactantis water. In various arrangements, the reduction reactants can be any ofcarbon dioxide, carbon monoxide, and combinations thereof, and/or anyother suitable reduction reactants. In one arrangement, the reductionreactant is carbon dioxide.

EXAMPLES: AQUEOUS SALTS IN OPERATING MEA CELLS Improved Lifetime andFaradaic Yield

The addition of salt to the anode water can improve the Faradaic yield,lower the cell voltage, and decrease the performance decay rate. FIGS.13A and 13B are two carbon dioxide electrolyzer performance plots, onewith no salt in the anode water and one with 2 mM NaHCO₃ in the anodewater. Both electrolyzers employed a bipolar MEA and a gold catalyst(cathode). In this example, the addition of the NaHCO₃ salt improves thecell performance. The cell with no salt in the anode water has anaverage voltage of 3.86V and average CO Faradaic yield of 0.53 for thefirst hour at 0.5 A/cm² and decay rate of 144 mV/hour and 0.018 COFaradaic yield/hour for hours 2-5 at 500 mA/cm². In comparison, the cellwith 2 mM NaHCO₃ has an average voltage of 3.52 V and average COFaradaic yield of 0.936 for the first hour at 0.5 A/cm² and decay rateof 15.5 mV/hour and 0.001 CO Faradaic yield/hour for hours 2-5 at 500mA/cm².

The presence of salt was also demonstrated to have a performanceenhancing effect in Faradaic yield and voltage efficiency of methane andethylene producing CO₂ electrolyzer systems. In this example, thepresence of NaHCO₃ was shown to improve voltage from 5.19V to 3.86V at0.2 A/cm² with an improved total detectable CO₂ Faradaic yield (includesCO, CH₄ and C₂H₄) improvement from 1% to 38%. The cells employed abipolar MEA and a copper catalyst (cathode). See FIG. 14.

Small variations in salt concentration can have a large effect. In theplot below, different concentrations of NaHCO₃ were added to the anodewater of a 100 cm² CO₂ electrolysis cell. In this example, 6 mM NaHCO₃showed the largest performance improvement, with higher Faradaic yieldthan lower concentration (2 mM) or higher concentrations (8 and 10 mM).The optimal concentration of salt also depends on the size ofelectrolyzer. In this example, 2 mM NaHCO₃ gives the best performancefor a 25 cm² electrolyzer. All MEA cells employed a bipolar MEA and agold catalyst (cathode). See FIG. 15.

Similarly, in a copper-based catalyst system where the CO₂ electrolyzerconverts CO₂ into methane, ethylene, ethanol and other small chainhydrocarbons and derivative organic compounds, the impact of saltconcentration was observed. All MEA cells employed a bipolar MEA and acopper catalyst (cathode). The effect of KHCO₃ salt concentration wasscreened for optimizing the production of C2 hydrocarbon at levels of1.5 mM to 30 mM. A total of 70% hydrocarbon yield was seen withconcentration set at 3 mM KHCO₃. This setting was found to improveethanol yield (40%) and ethylene yield (24%) compared to lower andhigher concentrations. See FIG. 16.

Identity of the Salt Changes Product Selectivity

Changing the identity of the salt can change the product selectivity.For example, using a copper catalyst on the cathode and comparing theproduct selectivity of CO₂ electrolysis when 3 mM KHCO₃ or 3 mM NaHCO₃are present in the anode water, shows that the presence of KHCO₃improves the selectivity to ethanol and ethylene with minor improvementsto methane selectivity. In FIG. 17, when the anode water was changedfrom NaHCO₃ to KHCO₃ during a reaction the selectivity for methanedeclined from 40% to 11% while the selectivity for ethylene increasedfrom 20% to 35%. A lower sensitivity was shown to product selectivitywhen switching the anion from bicarbonate to sulfate. All cells in thisexample employed a bipolar MEA and a copper catalyst (cathode).

In FIG. 18, improved selectivity toward ethanol was also seen when usinga larger cation salt KHCO₃ versus the smaller cation NaHCO₃.

In some implementations, ion concentration will decrease during along-term run. The addition of extra salts or change the electrolytereservoir to fresh salt solution will help to recover the selectivityand voltage. FIGS. 19A and 19B (Table) illustrate the selectivity andvoltage improvement after fresh salt solution is added or replace theold solution in the anolyte reservoir. The selectivity improves in therange of 0.3-2%, while the voltage is lowered in the range of 10-100 mV.In this example, all cells employed a bipolar MEA and a gold catalyst(cathode). The salt composition was 2 mM NaHCO₃. In one test, the saltsolution was replenished by changing out solution directly.

A scan of salt concentration on the selectivity of a copper catalysttoward methane was done in the range of 1 mM to 30 mM NaHCO₃.Selectivity for methane was significantly impacted at a low currentdensity of 100 mA/cm² showing an increase from 55% to 73% methane at theexpense of hydrogen generation. Above 20 mM salt concentration theeffect of increasing the amount of salt did not appear to positivelyimpact performance. At 250 mA/cm² a similar improvement in yield wasseen from around 52% to 62% methane, and improvement in voltage from4.25 to 4.00 V with some decline above 20 mA/cm² salt concentration. SeeFIG. 20, which shows improved methane selectivity with increasing saltconcentration up to 20 mM NaHCO₃ in a bipolar MEA cell.

Various salts were tested for effect on ethylene selectivity atdifferent concentrations. An anion conducting polymer only MEA was usedwith a copper catalyst (cathode). Concentration dependence of potassiumbicarbonate salt was seen at 3 and 6 mM levels, with 33% ethylene fromcarbon monoxide yield at 6 mM. See FIG. 21A, which illustrates theeffect of potassium bicarbonate salt in anolyte for CORR ethylene yieldin anion conducting polymer only MEA. In contrast, potassium hydroxidesalt is shown to improve voltage for the same reaction, with lowervoltage seen as higher concentrations of KOH were used. Good performancefor ethylene yield was seen at the lower KOH concentrations of around0.1 and 0.01 M. See FIG. 21B, which illustrates the effect of potassiumhydroxide salt concentration in anolyte for CORR ethylene yield in anionconducting polymer only setup.

Other Embodiments

Although omitted for conciseness, embodiments of the system and/ormethod can include every combination and permutation of the varioussystem components and the various method processes, wherein one or moreinstances of the method and/or processes described herein can beperformed asynchronously (e.g., sequentially), concurrently (e.g., inparallel), or in any other suitable order by and/or using one or moreinstances of the systems, elements, and/or entities described herein.

As a person skilled in the art will recognize from the previous detaileddescription and from the figures and claims, modifications and changescan be made to the preferred embodiments of the invention withoutdeparting from the scope of this invention defined in the followingclaims.

1. A membrane electrode assembly (MEA) comprising: a cathode layercomprising a carbon oxide reduction catalyst that promotes reduction ofa carbon oxide; an anode layer comprising a catalyst that promotesoxidation of a water; a polymer electrolyte membrane (PEM) layerdisposed between, and in contact with, the cathode layer and the anodelayer; and salt ions from a salt solution that contacts the MEA, whereinthe salt in the salt solution has a concentration of at least about 10uM. 2.-26. (canceled)
 27. An electrochemical system configured toelectrolytically reduce a carbon oxide, the system comprising: (a) amembrane electrode assembly (MEA) comprising: (i) a cathode layercomprising a carbon oxide reduction catalyst that promotes reduction ofa carbon oxide, (ii) an anode layer comprising a catalyst that promotesoxidation of a water, and (iii) a polymer electrolyte membrane (PEM)layer disposed between, and in contact with, the cathode layer and theanode layer; and (b) a source of anode water comprising a salt having aconcentration of at least about 10 uM in the anode water, wherein thesource of anode water is connected to the MEA in a manner allowing theanode water to contact the anode layer and provide the salt to the MEA.28. The electrochemical system of claim 27, wherein the carbon oxide iscarbon dioxide and wherein the reduction catalyst comprises gold,silver, copper, or a combination thereof.
 29. The electrochemical systemof claim 27, wherein the carbon oxide is carbon monoxide and wherein thecarbon oxide reduction catalyst comprises gold, silver, copper, or acombination thereof.
 30. The electrochemical system of claim 27, whereinthe cathode layer comprises an anion conducting polymer.
 31. Theelectrochemical system of claim 27, wherein the anode layer comprises acation conducting polymer.
 32. The electrochemical system of claim 27,wherein the MEA is bipolar, having at least one layer of a cationconducting polymer, and at least one layer of an anion conductingpolymer.
 33. The electrochemical system of claim 27, wherein the PEMlayer comprises a polymer electrolyte layer and a cathode buffer layer.34. (canceled)
 35. (canceled)
 36. The electrochemical system of claim27, wherein the salt comprises alkali metal ions.
 37. Theelectrochemical system of claim 27, wherein the salt comprises an anionselected from the group consisting of phosphate, sulfate, carbonate,bicarbonate, and hydroxide.
 38. The electrochemical system claim 27,wherein the MEA is a bipolar MEA, wherein the carbon oxide reductioncatalyst comprises copper, and wherein the salt comprises (i) an alkalimetal cation, and (ii) a bicarbonate, a sulfate, or a hydroxide anion.39. (canceled)
 40. (canceled)
 41. The electrochemical system of claim38, wherein the MEA is configured to produce methane by reducing carbondioxide and/or carbon monoxide at the cathode layer, and wherein thesalt comprises sodium ions.
 42. The electrochemical system of claim 38,wherein the MEA is configured to produce one or more organic compoundshaving two or more carbon atoms by reducing carbon dioxide and/or carbonmonoxide at the cathode layer, and wherein the salt comprises ions ofpotassium, cesium, rubidium, or any combination thereof.
 43. Theelectrochemical system of claim 27, wherein the MEA is a bipolar MEA,wherein the carbon oxide reduction catalyst comprises gold, and whereinthe salt comprises (i) an alkali metal cation and (ii) a bicarbonate,hydroxide, or sulfate anion.
 44. (canceled)
 45. (canceled)
 46. Theelectrochemical system of claim 43, wherein the MEA is configured toproduce carbon monoxide by reducing carbon dioxide at the cathode layer,and wherein the salt comprises alkali metal ions.
 47. Theelectrochemical system of claim 43, wherein the MEA comprisessubstantially no transition metal ions.
 48. The electrochemical systemof claim 27, wherein all polymers in the MEA are anion conductingpolymers, and wherein the carbon oxide reduction catalyst comprisescopper, and wherein the salt comprises (i) an alkali metal cation and(ii) a bicarbonate or hydroxide anion.
 49. (canceled)
 50. (canceled) 51.The electrochemical system of claim 48, wherein the MEA is configured toproduce methane by reducing carbon dioxide and/or carbon monoxide at thecathode layer, and wherein the salt comprises sodium ions.
 52. Theelectrochemical system of claim 48, wherein the MEA is configured toproduce one or more organic compounds having two or more carbon atoms byreducing carbon dioxide and/or carbon monoxide at the cathode layer, andwherein the salt comprises ions potassium, cesium, rubidium, or anycombination thereof.
 53. The electrochemical system of claim 27, furthercomprising a recirculation loop connected to the MEA and configured torecover anode water from the MEA, store and/or treat recovered anodewater, and supply stored or treated anode water to the MEA.
 54. Theelectrochemical system of claim 53, wherein the recirculation loopcomprises a reservoir for storing the anode the water.
 55. Theelectrochemical system of claim 53, wherein the recirculation loopcomprises an anode water purification element configured to removeimpurities from the anode water.
 56. The electrochemical system of claim53, wherein the recirculation loop comprises an inlet for receivingpurified water.
 57. The electrochemical system of claim 53, wherein therecirculation loop is connected to the source of anode water.
 58. Theelectrochemical system of claim 53, further comprising a cathode waterconduit connected to the recirculation loop and configured to providethe recirculation loop with water recovered from a carbon oxide streamafter the carbon oxide stream has contacted the cathode layer of theMEA.
 59. The electrochemical system of claim 53, further comprising awater separator coupled to the cathode water conduit and configured toseparate cathode water from the carbon oxide stream.
 60. A method ofelectrolytically reducing a carbon oxide, the method comprising:providing a salt solution to a membrane electrode assembly (MEA)comprising (a) a cathode layer comprising a carbon oxide reductioncatalyst that promotes reduction of a carbon oxide; (b) an anode layercomprising a catalyst that promotes oxidation of a water; and (c) apolymer electrolyte membrane (PEM) layer disposed between, and incontact with, the cathode layer and the anode layer, wherein the saltsolution comprises at least about 10 uM of a salt; and electrolyticallyreducing a carbon oxide at the cathode of the MEA while the MEA is incontact with the salt solution. 61.-90. (canceled)